Method for manufacturing semiconductor device

ABSTRACT

As a display device has a higher definition, the number of pixels, gate lines, and signal lines are increased. When the number of the gate lines and the signal lines are increased, a problem of higher manufacturing cost, because it is difficult to mount an IC chip including a driver circuit for driving of the gate and signal lines by bonding or the like. A pixel portion and a driver circuit for driving the pixel portion are provided over the same substrate, and at least part of the driver circuit includes a thin film transistor using an oxide semiconductor interposed between gate electrodes provided above and below the oxide semiconductor. Therefore, when the pixel portion and the driver portion are provided over the same substrate, manufacturing cost can be reduced.

TECHNICAL FIELD

The present invention relates to a semiconductor device using an oxidesemiconductor and a method for manufacturing the semiconductor device.

BACKGROUND ART

A thin film transistor formed over a flat plate such as a glasssubstrate is manufactured using amorphous silicon or polycrystallinesilicon, as typically seen in a liquid crystal display device. A thinfilm transistor formed using amorphous silicon has low electric fieldeffect mobility, but such a transistor can be formed over a glasssubstrate with a larger area. On the other hand, a thin film transistorformed using crystalline silicon has high electric field effectmobility, but a crystallization process such as laser annealing isnecessary and such a transistor is not always suitable for a largerglass substrate.

In view of the foregoing, attention has been drawn to a technique bywhich a thin film transistor is formed using an oxide semiconductor, andsuch a transistor is applied to an electronic device or an opticaldevice. For example, Patent Document 1 and Patent Document 2 disclose atechnique by which a thin film transistor is manufactured using zincoxide or an In—Ga—Zn—O-based oxide semiconductor as an oxidesemiconductor film and such a transistor is used as a switching elementor the like of an image display device.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

DISCLOSURE OF INVENTION

The electron field effect mobility of a thin film transistor in which achannel formation region is provided in an oxide semiconductor is higherthan that of a thin film transistor using amorphous silicon. The oxidesemiconductor film can be formed by a sputtering method or the like at atemperature of 300° C. or lower. Its manufacturing process is easierthan that of a thin film transistor using polycrystalline silicon.

Such an oxide semiconductor is expected to be used for forming a thinfilm transistor over a glass substrate, a plastic substrate, or thelike, and to be applied to a display device such as a liquid crystaldisplay device, an electroluminescent display device, or electronicpaper.

When the size of a display region of a display device is increased, thenumber of pixels is increased and thus the number of gate lines andsignal lines is increased. In addition, as a display device has a higherdefinition, the number of pixels is increased and thus the number ofgate lines and signal lines is increased. When the number of the gatelines and the signal lines is increased, it is difficult to mount ICchips including driver circuits for driving of the gate lines and thesignal lines by bonding or the like, whereby manufacturing cost isincreased.

Therefore, it is an object to reduce manufacturing cost by employing athin film transistor using an oxide semiconductor in at least part of adriver circuit for driving a pixel portion.

In the case of employing a thin film transistor using an oxidesemiconductor in at least part of a driver circuit for driving a pixelportion, high dynamic characteristics (on characteristics or frequencycharacteristics (referred to as f characteristics)) are required for thethin film transistor. It is another object to provide a thin filmtransistor having high dynamic characteristics (on characteristics) andto provide a driver circuit which enables high speed operation.

In addition, it is an object of an embodiment of the present inventionto provide a semiconductor device provided with a highly reliable thinfilm transistor in which an oxide semiconductor layer is used for achannel.

Gate electrodes are provided above and below an oxide semiconductorlayer to realize improvement of on characteristics and reliability of athin film transistor.

Further, by controlling gate voltage applied to the upper and lower gateelectrodes, threshold voltage can be controlled. The upper and lowergate electrodes may be electrically connected to each other so as tohave the same potential, or the upper and lower gate electrodes may beconnected to different wirings so as to have different potentials. Forexample, when the threshold voltage is set at 0 or close to 0 to reducedriving voltage, reduction of power consumption can be achieved.Alternatively, when the threshold voltage is set positive, the thin filmtransistor can function as an enhancement type transistor. Furtheralternatively, when the threshold voltage is set negative, the thin filmtransistor can function as a depletion type transistor.

For example, an inverter circuit including a combination of theenhancement type transistor and the depletion type transistor(hereinafter, such a circuit is referred to as an EDMOS circuit) can beused for a driver circuit. The driver circuit includes at least a logiccircuit portion, and a switch portion or a buffer portion. The logiccircuit portion has a circuit structure including the above EDMOScircuit. Further, a thin film transistor by which large on current canflow is preferably used for the switch portion or the buffer portion. Adepletion type transistor or a thin film transistor including gateelectrodes above and below an oxide semiconductor layer is used.

Thin film transistors having different structures can be formed over thesame substrate without greatly increasing the number of steps. Forexample, an EDMOS circuit using a thin film transistor including gateelectrodes above and below the oxide semiconductor layer may be formedfor the driver circuit for high-speed driving, and a thin filmtransistor including a gate electrode only below the oxide semiconductorlayer may be used for a pixel portion.

Note that an n-channel TFT whose threshold voltage is positive isreferred to as an enhancement type transistor, and an n-channel TFTwhose threshold voltage is negative is referred to as a depletion typetransistor, throughout this specification.

Examples of a material for the gate electrode provided above the oxidesemiconductor layer include an element selected from aluminum (Al),copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum(Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), and an alloycontaining any of the above elements as its component, and anyconductive film can be used without particular limitation. Further, thegate electrode is not limited to a single layer structure containing anyof the above elements, and can have a stacked structure of two or morelayers.

As a material for the gate electrode provided above the oxidesemiconductor layer, the same material as a pixel electrode can be used(a transparent conductive film or the like can be used in a case of atransmissive display device). For example, the gate electrode providedabove the oxide semiconductor layer can be formed in the same step as astep for forming the pixel electrode which is electrically connected tothe thin film transistor in the pixel portion. Consequently, the thinfilm transistor provided with the gate electrodes above and below theoxide semiconductor layer can be formed without greatly increasing thenumber of steps. In addition, by providing the gate electrode above theoxide semiconductor layer, in a bias-temperature stress test(hereinafter, referred to as a BT test) for examining reliability of athin film transistor, the amount of change in threshold voltage of thethin film transistor between before and after the BT test can bereduced. That is, provision of the gate electrode above the oxidesemiconductor layer can improve the reliability.

One embodiment of the present invention disclosed in this specificationis a semiconductor device including a first gate electrode over aninsulating surface; a first insulating layer over the first gateelectrode; an oxide semiconductor layer over the first insulating layer;a source electrode and a drain electrode over the oxide semiconductorlayer; a second insulating layer covering the source electrode and thedrain electrode; a second gate electrode over the second insulatinglayer, wherein the oxide semiconductor layer has a region whosethickness is smaller than that of a region overlapping with the sourceelectrode or the drain electrode, and the second insulating layer is incontact with the region whose thickness is smaller in the oxidesemiconductor layer.

The above-described structure can achieve at least one of theabove-described objects.

In the above-described structure, a width of the second gate electrodeis made larger than a width of the first gate electrode, whereby a gatevoltage can be applied to the whole oxide semiconductor layer from thesecond gate electrode.

Alternatively, in the above-described structure, when a width of thefirst gate electrode is made smaller than a width of the second gateelectrode, an area of the first gate electrode, which overlaps with thesource electrode and the drain electrode is reduced, so that parasiticcapacitance can be reduced. Further alternatively, a width of the firstgate electrode is made larger than the region whose thickness is smallerin the oxide semiconductor layer while a width of the second gateelectrode is made smaller than the region whose thickness is smaller inthe oxide semiconductor, so that the second gate electrode is notoverlapped with the source electrode or the drain electrode to reducethe parasitic capacitance more.

Another embodiment of the present invention is a semiconductor deviceincluding a pixel portion and a driver circuit, the pixel portionincludes at least a first thin film transistor having a first oxidesemiconductor layer, the driver circuit includes an EDMOS circuit inwhich at least a second thin film transistor having a second oxidesemiconductor layer and a third thin film transistor having a thirdoxide semiconductor layer, and the third thin film transistor includes afirst gate electrode below the third oxide semiconductor layer and asecond gate electrode above the third oxide semiconductor layer.

In the above-described structure, when the first thin film transistor inthe pixel portion is electrically connected to a pixel electrode and thepixel electrode is of the same material as the second gate electrode inthe driver circuit, the semiconductor device can be manufactured withoutincreasing the number of steps.

In the above-described structure, when the first thin film transistor inthe pixel portion is electrically connected to a pixel electrode and thepixel electrode is formed of a different material from the second gateelectrode in the driver circuit, for example, when the pixel electrodeis formed of a transparent conductive film and the second gate electrodeis formed of an aluminum film, resistance of the second gate electrodein the driver circuit can be reduced.

Further, a so-called dual-gate structure is provided, in which the thirdoxide semiconductor layer of the driver circuit overlaps with the firstgate electrode with the first insulating layer therebetween and alsooverlaps with the second gate electrode with the second insulating layertherebetween.

As a semiconductor device having a driver circuit, besides a liquidcrystal display device, a light-emitting display device using alight-emitting element and a display device using an electrophoreticdisplay element, which is also referred to electronic paper, can begiven.

Note that the term “display device” in this specification means an imagedisplay device, a light-emitting device, or a light source (including alighting device). Further, the “display device” includes the followingmodules in its category: a module including a connector such as anflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP) attached; a module having a TAB tape or aTCP which is provided with a printed wiring board at the end thereof;and a module having an integrated circuit (IC) which is directly mountedon a display element by a chip on glass (COG) method.

In a light-emitting display device using a light-emitting element, aplurality of thin film transistors are included in a pixel portion, anda portion in which a gate electrode of a thin film transistor iselectrically connected to a source wiring or a drain wiring of anothertransistor is included in the pixel portion.

Since a thin film transistor is easily broken due to static electricityor the like, a protective circuit for protecting the driver circuit ispreferably provided over the same substrate for a gate line or a sourceline. The protective circuit is preferably formed with a non-linearelement including an oxide semiconductor.

The oxide semiconductor used in this specification is a thin filmexpressed by InMO₃(ZnO)_(m) (m>0), and a thin film transistor using thethin film as a semiconductor layer is formed. Note that M denotes onemetal element or a plurality of metal elements selected from Ga, Fe, Ni,Mn, and Co. For example, M denotes Ga in some cases; meanwhile, Mdenotes the above metal element such as Ni or Fe in addition to Ga (Gaand Ni or Ga and Fe) in other cases. Further, the above oxidesemiconductor may contain Fe or Ni, another transitional metal element,or an oxide of the transitional metal as an impurity element in additionto the metal element contained as M. In this specification, this thinfilm is also referred to as an In—Ga—Zn—O-based non-single-crystal film.

The In—Ga—Zn—O-based non-single-crystal film is formed by a sputteringmethod, and is heated at 200° C. to 500° C., typically 300° C. to 400°C., for 10 to 100 minutes. Note that an amorphous structure is observedby XRD analysis as the crystal structure of the In—Ga—Zn—O-basednon-single-crystal film analyzed.

An oxide semiconductor typified by the In—Ga—Zn—O-basednon-single-crystal film is a material having a wide energy gap (Eg);therefore, even if two gate electrodes are provided above and below anoxide semiconductor layer, increase of off current can be suppressed.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

By forming the thin film transistor using the oxide semiconductorinterposed between the two gate electrodes provided above and below theoxide semiconductor in a peripheral circuit such as a gate line drivercircuit or a source line driver circuit, or a pixel portion,manufacturing cost is reduced.

With the thin film transistor using the oxide semiconductor interposedbetween the two gate electrodes provided above and below the oxidesemiconductor, in a BT test, the amount of change in threshold voltageof the thin film transistor between before and after the BT test can bereduced. That is, the thin film transistor includes the oxidesemiconductor interposed between the two gate electrodes provided aboveand below the oxide semiconductor, whereby reliability of the thin filmtransistor can be improved.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A, 1B, and 1C are cross-sectional views illustrating an exampleof a display device of Embodiment 1, another example of a display deviceof Embodiment 1, and another example of a display device of Embodiment1, respectively;

FIGS. 2A, 2B, and 2C are a cross-sectional view, an equivalent circuitdiagram, and a top view of a semiconductor device of Embodiment 2,respectively;

FIGS. 3A and 3B are block diagrams entirely illustrating display devicesof Embodiment 3;

FIG. 4 is a diagram illustrating arrangement of a wiring, an inputterminal, and the like in a display device of Embodiment 3;

FIG. 5 is a block diagram illustrating a structure of a shift registercircuit;

FIG. 6 is a diagram illustrating an example of a flip-flop circuit;

FIG. 7 is a view illustrating a layout view (a top view) of theflip-flop circuit;

FIG. 8 is a diagram illustrating a timing chart for showing operation ofa shift register circuit;

FIGS. 9A to 9C are views illustrating a method for manufacturing asemiconductor device of Embodiment 4;

FIGS. 10A to 10C are views illustrating a method for manufacturing thesemiconductor device of Embodiment 4;

FIG. 11 is a view illustrating a method for manufacturing thesemiconductor device of Embodiment 4;

FIG. 12 is a view illustrating a method for manufacturing thesemiconductor device of Embodiment 4;

FIG. 13 is a view illustrating a method for manufacturing thesemiconductor device of Embodiment 4.

FIG. 14 is a view illustrating the semiconductor device of Embodiment 4;

FIGS. 15A, 15B, 15C, and 15D are views illustrating semiconductordevices of Embodiment 4;

FIG. 16 is a view illustrating the semiconductor device of Embodiment 4;

FIG. 17 is a cross-sectional view illustrating a semiconductor device ofEmbodiment 5;

FIG. 18 is a diagram illustrating a pixel equivalent circuit of asemiconductor device of Embodiment 6;

FIGS. 19A to 19C are cross-sectional views illustrating semiconductordevices of Embodiment 6;

FIGS. 20A and 20B are a top view and a cross-sectional view illustratinga semiconductor device of Embodiment 6, respectively;

FIGS. 21A and 21B are top views and FIG. 21C is a cross-sectional viewillustrating a semiconductor device of Embodiment 7;

FIG. 22 is a cross-sectional view illustrating a semiconductor device ofEmbodiment 7;

FIGS. 23A to 23D are external views illustrating examples of electronicdevices;

FIGS. 24A and 24B are external views illustrating examples of atelevision device and a digital photo frame, respectively;

FIGS. 25A and 25B are external views illustrating examples of mobilephones; and

FIG. 26 is a cross-sectional view illustrating a semiconductor device ofEmbodiment 9.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described below. The present invention is notlimited to the description below and it is easily understood by thoseskilled in the art that the mode and details can be changed variouslywithout departing from the scope and spirit of the present invention.Therefore, the present invention should not be interpreted as beinglimited to description in the embodiments below.

Embodiment 1

FIG. 1A illustrates an example in which a first thin film transistor 430used for a driver circuit and a second thin film transistor 170 used fora pixel portion are provided over the same substrate. Note that FIG. 1Ais also an example of a cross-sectional view of a display device.

The pixel portion and the driver circuit are formed over the samesubstrate. In the pixel portion, the second thin film transistors 170,which are enhancement type transistors, arranged in a matrix form areeach used for switching on/off of voltage application to a pixelelectrode 110. The second thin film transistor 170 arranged in the pixelportion is formed using an oxide semiconductor layer 103. As forelectric characteristics of the second thin film transistor, on/offratio is 10⁹ or more at a gate voltage ±20 V; therefore, displaycontrast can be improved, and further, leakage current is small, wherebylow power consumption driving can be realized. The on/off ratio is aratio of on current to off current (I_(on)/I_(off)), and the higher thevalue of the I_(on)/I_(off) is, the better switching characteristics is.Thus, high on/off ratio contributes to improvement of display contrast.Note that on current is current which flows between a source electrodeand a drain electrode when a transistor is in an on state. Meanwhile,off current is current which flows between the source electrode and thedrain electrode when the transistor is in an off state. For example, inan n-channel transistor, the off current is current which flows betweena source electrode and a drain electrode when gate voltage is lower thanthreshold voltage of the transistor. Therefore, an enhancement typetransistor is preferably used for the pixel portion to achieve highcontrast and low-power-consumption driving.

In the driver circuit, at least one thin film transistor 430 including afirst gate electrode 401 below an oxide semiconductor layer 405 and asecond gate electrode 470 above the oxide semiconductor layer 405 isused. The second gate electrode 470 can also be called a back-gateelectrode. When the back-gate electrode is formed, in a bias-temperaturestress test (hereinafter, referred to as a BT test) for examiningreliability of a thin film transistor, the amount of change in thresholdvoltage of the thin film transistor between before and after the BT testcan be reduced.

A structure of this thin film transistor 430 is described with referenceto FIG. 1A. The first gate electrode 401 provided over a substrate 400having an insulating surface is covered with a first gate insulatinglayer 403, and the oxide semiconductor layer 405 is provided over thefirst gate insulating layer 403 overlapping with the first gateelectrode 401. Over the oxide semiconductor layer 405, a first wiring409 and a second wiring 410 are provided. The oxide semiconductor layer405 includes a region whose thickness is smaller than the thickness of aregion which serves as a source electrode or a drain electrode andoverlaps with the first wiring 409 and the second wiring 410. A secondgate insulating layer 412 is provided so as to be over and in contactwith the region whose thickness is smaller in the oxide semiconductorlayer 405. Further, the second gate electrode 470 is provided over thesecond gate insulating layer 412.

The oxide semiconductor layer 405 is formed, e.g., at argon gas flowrate of 10 sccm and oxygen flow rate of 5 sccm by a sputtering methodusing a target wherein In₂O₃:Ga₂O₃:ZnO=1:1:1 (In:Ga:Zn=1:1:0.5). Inaddition, an n⁺ layer 406 a is provided between the oxide semiconductorlayer 405 and the first wiring 409, and an n⁺ layer 406 b is providedbetween the oxide semiconductor layer 405 and the second wiring 410.

In this embodiment, the n⁺ layers 406 a and 406 b serving as source anddrain regions are In—Ga—Zn—O-based non-single-crystal films, which areformed under deposition conditions different from the depositionconditions of the oxide semiconductor layer 405, and are oxidesemiconductor layers having lower resistance. For example, the n⁺ layers406 a and 406 b formed of oxide semiconductor layers obtained at argongas flow rate of 40 sccm have n-type conductivity and activation energy(ΔE) of from 0.01 eV to 0.1 eV. Note that in this embodiment, the n⁺layers 406 a and 406 b are In—Ga—Zn—O-based non-single-crystal films,which include at least amorphous component. The n⁺ layers 406 a and 406b include crystal grains (nano crystals) in the amorphous component insome cases. The diameter of the crystal grains (nano crystals) includedin the n⁺ layers 406 a and 406 b is about 1 nm to 10 nm, typically about2 nm to 4 nm.

Further, the first gate electrode 401 and the second gate electrode 470may be electrically connected to each other so as to have the samepotential. When the first gate electrode 401 and the second gateelectrode 470 have the same potential, gate voltage can be applied fromupper and lower sides of the oxide semiconductor layer, so that theamount of current which flows in an on state can be increased.

Further, by electrically connecting a control signal line for shiftingthe threshold voltage to a negative value to either the first gateelectrode 401 or the second gate electrode 470, a depletion type TFT canbe formed.

Alternatively, by electrically connecting a control signal line forshifting the threshold voltage to a positive value to either the firstgate electrode 401 or the second gate electrode 470, an enhancement typeTFT can be formed.

Further, there is no particular limitation on a combination of two thinfilm transistors used for the driver circuit, and a combination of athin film transistor including one gate electrode as the depletion typeTFT and a thin film transistor including two gate electrodes as theenhancement type TFT may be employed. In that case, a thin filmtransistor in the pixel portion has a structure in which gate electrodesare provided above and below the oxide semiconductor layer.

Alternatively, the thin film transistor in the pixel portion may have astructure in which gate electrodes are provided above and below theoxide semiconductor layer, and the enhancement type TFT and thedepletion type TFT in the driver circuit may each have a structure inwhich gate electrodes are provided above and below the oxidesemiconductor layer. In that case, a structure in which a control signalline for controlling the threshold voltage is electrically connected toeither of the upper and lower gate electrodes and the connected gateelectrode controls the threshold voltage is employed.

Note that in FIG. 1A, the second gate electrode 470 is formed of thesame material as the pixel electrode 110 in the pixel portion, forexample, using a transparent conductive film in a transmissive liquidcrystal display device, in order to reduce the number of steps. However,there is no particular limitation on the second gate electrode 470. Inaddition, the example in which a width of the second gate electrode 470is larger than a width of the first gate electrode 401 and also largerthan a width of the oxide semiconductor layer is illustrated; however,there is no particular limitation on the width of the second gateelectrode 470. Note that the width of the first gate electrode 401 islarger than that of the region whose thickness is smaller in the oxidesemiconductor layer.

FIG. 1B illustrates an example different from FIG. 1A in the materialand the width of the second gate electrode. Further, FIG. 1B is also anexample of a display device in which the second thin film transistor 170connected to an organic light-emitting element or an inorganiclight-emitting element is included in the pixel portion.

In FIG. 1B, as a material for an electrode 471 which functions as asecond gate electrode of a thin film transistor 432, a metal material(an element selected from aluminum (Al), copper (Cu), titanium (Ti),tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium(Nd), and scandium (Sc), or an alloy containing any of theabove-described elements as its component) is used. A width of theelectrode 471 in the cross section is smaller than that of the secondgate electrode 470 in FIG. 1A. Further, the width of the electrode 471is smaller than a width of the oxide semiconductor layer. By reducingthe width of the electrode 471, the overlapping area of the electrode471 with the first wiring 409 and the second wiring 410 with the secondgate insulating layer 412 therebetween can be reduced, so that parasiticcapacitance can be reduced. Note that in FIG. 1B, the width of theelectrode 471 is larger than that of the region whose thickness issmaller in the oxide semiconductor layer.

The light-emitting element includes at least a first electrode 472, alight-emitting layer 475, and a second electrode 474. In FIG. 1B, theelectrode 471 is formed of the same material as the first electrode 472in the pixel portion, for example, using aluminum or the like, in orderto reduce the number of steps; however, there is no particularlimitation on the electrode 471. Further, in FIG. 1B, an insulatinglayer 473 functions as a partition for insulating the first electrodesof the adjacent pixels from each other.

Further, FIG. 1C illustrates an example different from FIG. 1A in thematerial and the width of the second gate electrode. In FIG. 1C, as amaterial for an electrode 476 which functions as a second gate electrodeof a thin film transistor 433, a metal material (an element selectedfrom aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten(W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc),or an alloy containing any of the above-described elements as itscomponent) is used. A width of the second gate electrode in a crosssection is smaller than that in FIG. 1B. When the width is still smallerthan that in FIG. 1B, it is possible to form the second gate electrodeso as not to overlap with the first wiring 409 and the second wiring 410with the second gate insulating layer 412 therebetween, and thus theparasitic capacitance can further be reduced. The width of the electrode476 illustrated in FIG. 1C is smaller than that of the region whosethickness is smaller in the oxide semiconductor. In forming theelectrode 476 having such a small width, a process using wet etching orthe like is preferably performed so that both ends of the electrode 476are positioned on an inner portion than end portions of a resist mask.However, in FIG. 1C, since a metal material different from that of thepixel electrode 110 is used, one more photolithography process is addedto form the electrode 476, and one more mask is needed.

By using the thin film transistor including the oxide semiconductorinterposed between the two gate electrodes above and below the oxidesemiconductor for a peripheral circuit such as a gate line drivercircuit or a source line driver circuit, or a pixel portion, which isused for a liquid crystal display device, a light-emitting displaydevice, or electronic paper, high speed driving or low power consumptioncan be achieved. Further, both the pixel portion and the driver circuitcan be provided over the same substrate without greatly increasing thenumber of steps. By providing various circuits in addition to the pixelportion over the same substrate, manufacturing cost of a display devicecan be reduced.

Embodiment 2

Although one thin film transistor has been described as the thin filmtransistor in the driver circuit in Embodiment 1, an example of formingan inverter circuit of a driver circuit with use of two n-channel thinfilm transistors will be described below in Embodiment 2. A thin filmtransistor illustrated in FIG. 2A is the same as the thin filmtransistor 430 illustrated in FIG. 1A of Embodiment 1; therefore, thesame parts are denoted by the same reference numerals.

The driver circuit for driving a pixel portion is formed using aninverter circuit, a capacitor, a resistor, and the like. When theinverter circuit is formed using two n-channel TFTs in combination,there are an inverter circuit having a combination of an enhancementtype transistor and a depletion type transistor (hereinafter, referredto as an EDMOS circuit) and an inverter circuit having a combination oftwo enhancement type TFTs (hereinafter, referred to as an EEMOScircuit).

A cross-sectional structure of the inverter circuit of the drivercircuit is illustrated in FIG. 2A. Note that the thin film transistor430 and a second thin film transistor 431 illustrated in FIG. 2A to 2Care bottom-gate thin film transistors, and are examples of thin filmtransistors in which wirings are provided over a semiconductor layerwith a source region and a drain region therebetween.

In FIG. 2A, the first gate electrode 401 and a gate electrode 402 areprovided over the substrate 400. The first gate electrode 401 and thegate electrode 402 can be formed to have a single-layer structure or astacked structure using a metal material such as molybdenum, titanium,chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandiumor an alloy material containing any of these as its main component.

As a two-layer stacked structure of the first gate electrode 401 and thegate electrode 402, for example, a two-layer stacked structure in whicha molybdenum layer is stacked over an aluminum layer, a two-layerstructure in which a molybdenum layer is stacked over a copper layer, atwo-layer structure in which a titanium nitride layer or a tantalumnitride layer is stacked over a copper layer, or a two-layer structurein which a titanium nitride layer and a molybdenum layer are stacked ispreferable. As a three-layer stacked structure, stacked layers of atungsten layer or a tungsten nitride layer, an alloy of aluminum andsilicon or an alloy of aluminum and titanium, and a titanium nitridelayer or a titanium layer is preferable.

In addition, over the first gate insulating layer 403 covering the firstgate electrode 401 and the gate electrode 402, an oxide semiconductorlayer 405 and a second oxide semiconductor layer 407 are provided.

A first wiring 409 and a second wiring 410 are provided over the oxidesemiconductor layer 405, and the second wiring 410 is directly connectedto the gate electrode 402 through a contact hole 404 formed in the firstgate insulating layer 403. Further, a third wiring 411 is provided overthe second oxide semiconductor layer 407.

The thin film transistor 430 includes the first gate electrode 401 andthe oxide semiconductor layer 405 overlapping with the first gateelectrode 401 with the first gate insulating layer 403 therebetween. Thefirst wiring 409 is a power supply line to which negative voltage VDL isapplied (a negative power supply line). This power supply line may be apower supply line with a ground potential (a ground potential powersupply line).

Further, the second thin film transistor 431 includes the gate electrode402 and the second oxide semiconductor layer 407 overlapping with thegate electrode 402 with the first gate insulating layer 403 interposedtherebetween. The third wiring 411 is a power supply line to whichpositive voltage VDH is applied (a positive power supply line).

In addition, an n⁺ layer 408 a is provided between the second oxidesemiconductor layer 407 and the second wiring 410, and an n⁺ layer 408 bis provided between the second oxide semiconductor layer 407 and thethird wiring 411.

Further, a top view of the inverter circuit of the driver circuit isillustrated in FIG. 2C. In FIG. 2C, a cross-section taken along a chainline Z1-Z2 corresponds to FIG. 2A.

Further, an equivalent circuit of the EDMOS circuit is illustrated inFIG. 2B. A circuit connection illustrated in FIG. 2A corresponds to thatin FIG. 2B, and is an example in which the thin film transistor 430 isan enhancement type n-channel transistor while the second thin filmtransistor 431 is a depletion type n-channel transistor.

In this embodiment, in order that the thin film transistor 430 can serveas an enhancement type n-channel transistor, the second gate insulatinglayer 412 is provided over the oxide semiconductor layer 405 and thesecond gate electrode 470 is provided over the second gate insulatinglayer 412 so that threshold value of the thin film transistor 430 iscontrolled by voltage applied to the second gate electrode 470.

Further, the second gate insulating layer 412 also functions as aprotective layer covering the second oxide semiconductor layer 407.

Note that the example in which the second wiring 410 is directlyconnected to the gate electrode 402 through the contact hole 404 formedin the first gate insulating layer 403 is illustrated in FIGS. 2A and2C, but, without particular limitations, a connection electrode may beseparately provided, thereby electrically connecting the second wiring410 and the gate electrode 402.

Further, this embodiment mode can be freely combined with Embodiment 1.

Embodiment 3

In Embodiment 3, a display device will be described with reference toblock diagrams and the like.

FIG. 3A illustrates an example of a block diagram of an active matrixliquid crystal display device. The liquid crystal display deviceillustrated in FIG. 3A includes, over a substrate 300, a pixel portion301 having a plurality of pixels each provided with a display element; ascan line driver circuit 302 which controls a scan line connected to agate electrode of each pixel; and a signal line driver circuit 303 whichcontrols video signal input to a selected pixel.

FIG. 3B illustrates an example of a block diagram of an active matrixlight-emitting display device. The light-emitting display deviceillustrated in FIG. 3B includes, over a substrate 310, a pixel portion311 having a plurality of pixels each provided with a display element; afirst scan line driver circuit 312 and a second scan line driver circuit313, each of which controls a scan line connected to a gate electrode ofa pixel; and a signal line driver circuit 314 which controls videosignal input to a selected pixel. In a case where two TFTs (thin filmtransistor) of a switching TFT and a current controlling TFT arearranged in one pixel, in the light-emitting display device illustratedin FIG. 3B, a signal which is input to a first scan line connected to agate electrode of the switching TFT is generated in the first scan linedriver circuit 312, and a signal which is input to a second scan lineconnected to a gate electrode of the current controlling TFT isgenerated in the second scan line driver circuit 313. Note that astructure in which the signal input to the first scan line and thesignal input to the second scan line are generated in one scan linedriver circuit may also be employed. Alternatively, for example, aplurality of the first scan lines used for controlling operation of aswitching element may be provided in each pixel, depending on the numberof TFTs included in the switching element. In this case, all signalswhich are input to the plurality of the first scan lines may begenerated in one scan line driver circuit, or may be generatedseparately by a plurality of scan line driver circuits.

Note that modes in which the scan line driver circuit 302, the firstscan line driver circuit 312, the second scan line driver circuit 313,and the signal line driver circuits 303 and 314 are formed in thedisplay device are described here; however, part of the scan line drivercircuit 302, the first scan line driver circuit 312, or the second scanline driver circuit 313 may be mounted with use of a semiconductordevice such as an IC. Alternatively, part of the signal line drivercircuits 303 or 314 may be mounted with a semiconductor device such asan IC.

FIG. 4 is a diagram illustrating a positional relation between a pixelportion and a protective circuit including a signal input terminal 321,a scan line 323, a signal line 324, and a non-linear element, whichconstitute the display device. A pixel portion 327 includes scan lines323 and signal lines 324 which are arranged over a substrate 320 havingan insulating surface so as to intersect with each other. Note that thepixel portion 327 corresponds to the pixel portion 301 and the pixelportion 311 illustrated in FIGS. 3A and 3B.

The pixel portion 301 is connected to the signal line driver circuit 303by a plurality of signal lines S1 to Sm (not illustrated) which arearranged in columns and extended from the signal line driver circuit303, and connected to the scan line driver circuit 302 by a plurality ofscan lines G1 to Gn (not illustrated) which are arranged in rows andextended from the scan line driver circuit 302. The pixel portion 301includes a plurality of pixels (not illustrated) arranged in a matrixform by the signal lines S1 to Sm and the scan lines G1 to Gn. Then,each pixel is connected to a signal line Sj (any one of the signal linesS1 to Sm) and a scan line Gi (any one of the scan lines G1 to Gn).

The pixel portion 327 includes a plurality of pixels 328 arranged in amatrix form. The pixel 328 includes a pixel TFT 329 connected to thescan line 323 and the signal line 324, a storage capacitor 330, and apixel electrode 331.

The pixel structure here illustrates a case where one electrode of thestorage capacitor 330 is connected to the pixel TFT 329 and the otherelectrode thereof is connected to a capacitor line 332. Further, thepixel electrode 331 serves as one electrode which drives a displayelement (a liquid crystal element, a light-emitting element, a contrastmedium (electronic ink), or the like). The other electrode of such adisplay element is connected to a common terminal 333.

Some protective circuits are provided between the pixel portion 327 andsignal line input terminals 322. In addition, other protective circuitsare provided between the scan line driver circuit and the pixel portion327. In this embodiment, a plurality of protective circuits are providedso that the pixel TFT 329 and the like are not broken when surge voltagedue to static electricity or the like is applied to the scan line 323,the signal line 324, and a capacitor bus line 337. Therefore, theprotective circuits are formed so that charge is released into a commonwiring when the surge voltage is applied.

In this embodiment, an example in which a protective circuit 334, aprotective circuit 335, and a protective circuit 336 are arranged on ascan line 323 side, on a signal line 324 side, and on a capacitor busline 337 side, respectively is illustrated. Note that an arrangementposition of the protective circuits is not limited thereto. In addition,in a case where the scan line driver circuit is not mounted with use ofa semiconductor device such as an IC, the protective circuit 334 is notnecessarily provided on the scan line 323 side.

By use of the TFT described in Embodiment 1 or Embodiment 2 for thesecircuits, the following advantages can be obtained.

The driver circuit is roughly divided into a logic circuit portion, anda switch portion or a buffer portion. A TFT provided in the logiccircuit portion preferably has a structure in which control thresholdvoltage can be controlled. On the other hand, a TFT provided in theswitch portion or the buffer portion preferably has large on current. Byprovision of a driver circuit including the TFTs described in Embodiment1 or Embodiment 2, the threshold voltage of the TFT provided in thelogic circuit portion can be controlled, and the on current of the TFTprovided in the switch portion or the buffer portion can be increased.Furthermore, the TFTs described in Embodiment 1 or Embodiment 2contribute to reducing an area occupied by the driver circuit andnarrowing a frame.

A shift register circuit included in the scan line driver circuit isdescribed below. A shift register circuit illustrated in FIG. 5 includesa plurality of flip-flop circuits 351, a control signal line 352, acontrol signal line 353, a control signal line 354, a control signalline 355, a control signal line 356, and a reset line 357.

As illustrated in the shift register circuit of FIG. 5, in the flip-flopcircuits 351, a start pulse SSP is input to an input terminal IN of thefirst stage through the control signal line 352, and an output signalterminal S_(out) of the flip-flop circuit 351 of the preceding stage isconnected to an input terminal IN of the next stage. Further, a resetterminal RES of the N-th stage (N is a natural number) is connected toan output signal terminal S_(out) of the flip-flop circuit of the(N+3)th stage through the reset line 357. When it is assumed that afirst clock signal CLK1 is input to a clock terminal CLK of theflip-flop circuit 351 of the N-th stage through the control signal line353, a second clock signal CLK2 is input to the clock terminal CLK ofthe flip-flop circuit 351 of the (N+1)th stage through the controlsignal line 354. A third clock signal CLK3 is input to the clockterminal CLK of the flip-flop circuit 351 of the (N+2)th stage throughthe control signal line 355. A fourth clock signal CLK4 is input to theclock terminal CLK of the flip-flop circuit 351 of the (N+3)th stagethrough the control signal line 356. Then the first clock signal CLK1 isinput to the clock terminal CLK of the flip-flop circuit 351 of the(N+4)th stage through the control signal line 353. In addition, theflip-flop circuit 351 of the N-th stage outputs an output SR_(out)N ofthe flip flop circuit of the N-th stage from a gate output terminalG_(out).

Note that connection between the flip-flop circuits 351, and a powersource and a power supply line is not illustrated; however, eachflip-flop circuit 351 is supplied with a power supply potential Vdd anda power supply potential GND through the power supply line.

Note that the power supply potential described in this specificationcorresponds to a potential difference when a reference potential is 0 V.Therefore, the power supply potential is also referred to as powersupply voltage, or the power supply voltage is referred to as the powersupply potential in some cases.

Note that in this specification, description that “A and B are connectedto each other” includes a case where A and B are electrically connectedto each other in addition to a case where A and B are directly connectedto each other. Here, the description that “A and B are electricallyconnected to each other” includes the following cases: when an objecthaving any electrical function exists between A and B, A and B havesubstantially the same potential via the object. Specifically, thedescription that “A and B are electrically connected to each other”includes cases where A and B are considered to have substantially thesame potential in light of circuit operation, e.g., a case where A and Bare connected through a switching element such as a TFT and electricitytransmission through the switching element causes A and B to havesubstantially the same potential, a case where A and B are connected viaa resistor and a potential difference between potentials generated atboth ends of the resistor does not affect operation of a circuitincluding A and B, and the like.

Next, FIG. 6 illustrates one mode of the flip-flop circuit 351 includedin the shift register circuit illustrated in FIG. 5. The flip-flopcircuit 351 illustrated in FIG. 6 includes a logic circuit portion 361and a switch portion 362. The logic circuit portion 361 includes TFTs363 to 368. Further, the switch portion 362 includes TFTs 369 to 372.Note that the logic circuit portion is a circuit for switching a signalthat is output to a switch portion, which is a circuit in the nextstage, in response to a signal that is input from an external portion.In addition, the switch portion is a circuit for switching on/off of aTFT which functions as a switch in response to a signal input from anexternal portion and a control circuit portion, and for outputtingcurrent depending on the size and the structure of the TFT.

In the flip-flop circuit 351, an input terminal IN is connected to agate terminal of the TFT 364 and a gate terminal of the TFT 367. A resetterminal RES is connected to a gate terminal of the TFT 363. A clockterminal CLK is connected to a first terminal of the TFT 369 and a firstterminal of the TFT 371. A power supply line through which the powersupply potential Vdd is supplied is connected to a first terminal of theTFT 364, and a gate terminal and a second terminal of the TFT 366. Apower supply line through which the power supply potential GND issupplied is connected to a second terminal of the TFT 363, a secondterminal of the TFT 365, a second terminal of the TFT 367, a secondterminal of the TFT 368, a second terminal of the TFT 370, and a secondterminal of the TFT 372. Further, a first terminal of the TFT 363, asecond terminal of the TFT 364, a first terminal of the TFT 365, a gateterminal of the TFT 368, a gate terminal of the TFT 369, and a gateterminal of the TFT 371 are connected to each other. A first terminal ofthe TFT 366 is connected to a gate terminal of the TFT 365, a firstterminal of the TFT 367, a first terminal of the TFT 368, a gateterminal of the TFT 370, and a gate terminal of the TFT 372. Inaddition, a gate output terminal G_(out) is connected to a secondterminal of the TFT 369 and a first terminal of the TFT 370. An outputsignal terminal S_(out) is connected to a second terminal of the TFT 371and a first terminal of the TFT 372.

Note that a case where the TFTs 363 to 372 are all n-channel TFTs isdescribed here.

Note that a TFT is an element having at least three terminals of a gate,a drain, and a source, and has a channel formation region between adrain region and a source region. Current can flow through the drainregion, the channel formation region, and the source region. Here, thesource and the drain may be exchanged with each other in some casesdepending on a structure, operation conditions of the TFT, or the like;therefore, it is difficult to determine which the source is or which thedrain is. Therefore, regions functioning as the source and the drain arenot referred to as a source and a drain but referred to, for example, asa first terminal and a second terminal, respectively, in some cases. Insuch a case, a terminal functioning as a gate is referred to as a gateterminal.

Next, FIG. 7 illustrates an example of a layout view of the flip-flopcircuit 351 illustrated in FIG. 6.

The flip-flop circuit of FIG. 7 includes a power supply line 381 throughwhich the power supply potential Vdd is supplied, a reset line 382, thecontrol signal line 353, the control signal line 354, the control signalline 355, the control signal line 356, a control signal line 383, apower supply line 384 through which the power supply potential GND issupplied, the logic circuit portion 361, and the switch portion 362. Thelogic circuit portion 361 includes the TFTs 363 to 368. The switchportion 362 includes the TFTs 369 to 372. In FIG. 7, a wiring connectedto the gate output terminal G_(out) and a wiring connected to the outputsignal terminal S_(out) are also illustrated.

FIG. 7 illustrates a semiconductor layer 385, a first wiring layer 386,a second wiring layer 387, a third wiring layer 388, and a contact hole389. Note that the first wiring layer 386 may be formed with a layer ofa gate electrode, the second wiring layer 387 may be formed with a layerof source and drain electrodes of a TFT, and the third wiring layer 388may be formed with a layer of a pixel electrode in the pixel portion.However, without being limited to this example, the third wiring layer388 may be formed as a layer different from the layer of the pixelelectrode for example.

Note that connections between circuit elements in FIG. 7 are asillustrated in FIG. 6. Note that FIG. 7 illustrates the flip-flopcircuit to which the first clock signal is input; therefore, connectionsto the control signal lines 354 to 356 are not illustrated.

In the layout view of the flip-flop circuit of FIG. 7, by controllingthreshold voltage of the TFT 366 or the TFT 367 included in the logiccircuit portion 361, an EDMOS circuit 373 can be formed. Typically, theEDMOS circuit 373 in which the TFT 366 is a depletion type and the TFT367 is an enhancement type is formed, and the TFTs 369 to 372 includedin the switch portion 362 are dual-gate TFTs or depletion type TFTs.Note that in FIG. 6, the TFT 366 and the TFT 367 in the EDMOS circuit373 are different from the TFTs in the EDMOS circuit illustrated inFIGS. 2A to 2C in a connection position of the gate electrode of thedepletion type TFT.

The TFT 366 or the TFT 367 is formed so as to be a dual-gate TFT and apotential of a back-gate electrode is controlled, so that a depletiontype TFT or an enhancement type TFT can be formed.

In FIG. 7, a control signal line 390 which has the same potential as aback-gate electrode for controlling the threshold voltage of the TFT 366is separately provided to form a depletion type. The TFT 366 is adual-gate TFT, and a potential of the back-gate electrode is differentfrom a potential of the power supply line 381 through which the powersupply potential Vdd that is applied to the gate electrode is supplied.

FIG. 7 illustrates an example in which the TFTs 369 to 372 are dual-gateTFTs and the back-gate electrodes and the gate electrodes have the samepotentials, and a potential of each of the back-gate electrodes is thesame potential as that of the power supply line through which the powersupply potential Vdd that is applied to the gate electrode is supplied.

In this manner, TFTs arranged in the pixel portion and the drivercircuit of the display device can be formed using only n-channel TFTs inwhich an oxide semiconductor layer is used.

Further, the TFT 366 in the logic circuit portion 361 is a TFT forsupplying current in response to the power supply potential Vdd. The TFT366 is formed to be a dual-gate TFT or a depletion type TFT to increasethe flowing current, whereby miniaturization of the TFT can be achievedwithout reducing performance.

Further, in the TFTs included in the switch portion 362, the amount ofcurrent flowing in the TFTs can be increased and switching of on/off canbe performed at high speed; therefore, an area occupied by the TFTs canbe reduced without reducing performance. Accordingly, an area occupiedby the circuit including the TFTs can also be reduced. Note that theTFTs 369 to 372 in the switch portion 362 may be formed to be dual-gateTFTs such that the semiconductor layer 385 is interposed between thefirst wiring layer 386 and the third wiring layer 388 as illustrated inthe drawing.

The example that the dual-gate TFTs each have a structure in which thesemiconductor layer 385 is interposed between the first wiring layer 386and the third wiring layer 388 which have the same potential by beingconnected to each other through the contact hole 389 is illustrated inFIG. 7. However, there is no particular limitation; for example, astructure in which a control signal line is separately provided for thethird wiring layer 388 to control a potential of the third wiring layer388 independently from the first wiring layer 386 may be employed.

Note that in the layout view of the flip-flop circuit illustrated inFIG. 7, the shapes of the channel formation regions of the TFTs 363 to372 may be U shapes (reversed C shapes or horseshoe shapes). Inaddition, although all the TFTs have the same size in FIG. 7, the sizeof each TFT which is connected to the output signal terminal S_(out) orthe gate output terminal G_(out) may be changed as appropriate inaccordance with the amount of a load of a subsequent stage.

Next, operation of the shift register circuit illustrated in FIG. 5 isdescribed with reference to a timing chart illustrated in FIG. 8. FIG. 8illustrates the start pulse SSP and the first to fourth clock signalsCLK1 to CLK4, which are supplied to the control signal lines 352 to 356illustrated in FIG. 5, respectively, and the Sout 1 to Sout 5 outputfrom the output signal terminals S_(out) of the flip-flop circuits ofthe first to fifth stages. Note that in description of FIG. 8, thereference numerals denoting the respective elements in FIG. 6 and FIG. 7are used.

Note that FIG. 8 is a timing chart in the case where each TFT includedin the flip-flop circuits is an n-channel TFT. Further, the first clocksignal CLK1 is shifted from the fourth clock signal CLK4 by ¼ wavelength(a section divided by dotted lines) as illustrated.

First, in a period T1, the start pulse SSP is input to the flip-flopcircuit of the first stage at an H level, and the logic circuit portion361 turns the TFTs 369 and 371 on and the TFTs 370 and 372 off in theswitch portion. At this time, since the first clock signal CLK1 is at anL level, the Sout 1 is at an L level.

Note that in the period T1, signals are not input to the IN terminals ofthe flip-flop circuits of the second and subsequent stages, so that theflip-flop circuits output L levels without operation. Note thatdescription is made assuming that each flip-flop circuit of the shiftregister circuit outputs an L level in an initial state.

Next, in a period T2, the logic circuit portion 361 controls the switchportion 362 in the flip-flop circuit of the first stage in a mannersimilar to the period T1. In the period T2, the first clock signal CLK1is at an H level, and thus the Sout 1 is at an H level. Further, in theperiod T2, the Sout 1 is input to the IN terminal of the flip-flopcircuit of the second stage at an H level, and the logic circuit portion361 turns the TFTs 369 and 371 on and the TFTs 370 and 372 off in theswitch portion. At this time, since the second clock signal CLK2 is atan L level, the Sout 2 is at an L level.

Note that in the period T2, signals are not input to the IN terminals ofthe flip-flop circuits of the third and subsequent stages, so that theflip-flop circuits output L levels without operation.

Next, in a period T3, the logic circuit portion 361 controls the switchportion 362 so that a state of the period T2 is held in the flip-flopcircuit of the first stage. Therefore, in the period T3, the first clocksignal CLK1 is at an H level and the Sout 1 is at an H level. Further,in the period T3, the logic circuit portion 361 controls the switchportion 362 in the flip-flop circuit of the second stage in a mannersimilar to the period T2. In the period T3, since the second clocksignal CLK2 is at an H level, the Sout 2 is at an H level. In addition,the Sout 2 is input to the IN terminal of the flip-flop circuit of thethird stage at an H level in the period T3, and the logic circuitportion 361 turns the TFTs 369 and 371 on and the TFTs 370 and 372 offin the switch portion. At this time, the third clock signal CLK3 is atan L level, and thus the Sout 3 is at an L level.

Note that in the period T3, signals are not input to the IN terminals ofthe flip-flop circuits of the fourth and subsequent stages, so that theflip-flop circuits output L levels without operation.

Next, in the period T4, the logic circuit portion 361 controls theswitch portion 362 so that a state of the period T3 is held in theflip-flop circuit of the first stage. Therefore, in the period T4, thefirst clock signal CLK1 is at an L level and the Sout 1 is at an Llevel. Further, in the period T4, the logic circuit portion 361 controlsthe switch portion 362 so that a state of the period T3 is held in theflip-flop circuit of the second stage. Therefore, in the period T4, thesecond clock signal CLK2 is at an H level and Sout 2 is at an H level.In addition, in the period T4, the logic circuit portion 361 controlsthe switch portion 362 in the flip-flop circuit of the third stage in amanner similar to the period T3. In the period T4, since the third clocksignal CLK3 is at an H level, the Sout 3 is at an H level. The Sout 3 isinput to the IN terminal of the flip-flop circuit of the fourth stage atan H level in the period T4, and the logic circuit portion 361 turns theTFTs 369 and 371 on and the TFTs 370 and 372 off in the switch portion362. At this time, since the fourth clock signal CLK4 is at an L level,the Sout 4 is at an L level.

Note that in the period T4, signals are not input to the IN terminals ofthe flip-flop circuits of the fifth and subsequent stages, so that theflip-flop circuits output L levels without operation.

Next, in a period T5, the logic circuit portion 361 controls the switchportion 362 so that a state of the period T3 is held in the flip-flopcircuit of the second stage. Therefore, in the period T5, the secondclock signal CLK2 is at an L level and the Sout 2 is at an L level.Further, in the period T5, the logic circuit portion 361 controls theswitch portion 362 so that a state of the period T4 is held in theflip-flop circuit of the third stage. Therefore, in the period T5, thethird clock signal CLK3 is at an H level and the Sout 3 is at an Hlevel. In addition, in the period T5, the logic circuit portion 361controls the switch portion 362 in the flip-flop circuit of the fourthstage in a manner similar to the period T4. In the period T5, since thefourth clock signal CLK4 is at an H level, the Sout 4 is at an H level.The flip-flop circuits of the fifth and subsequent stages have a wiringconnection and a timing of signals to be input similar to those of theflip-flop circuits of the first to fourth stages; therefore, descriptionthereof is omitted.

As illustrated in the shift register circuit of FIG. 5, the Sout 4 alsofunctions as a reset signal of the flip-flop circuit of the first stage.In the period T5, the Sout 4 is at an H level and this signal is inputto the reset terminal RES of the flip-flop circuit of the first stage.When the reset signal is input, the TFTs 369 and 371 are turned off andthe TFTs 370 and 372 are turned on in the switch portion 362. Then, theSout 1 of the flip-flop circuit of the first stage outputs an L leveluntil input of the next start pulse SSP.

By the above-described operation, in the flip-flop circuits of thesecond and subsequent stages, the logic circuit portions are also resetbased on the reset signals which are output from the flip-flop circuitsof subsequent stages. As shown by the Souts 1 to 5, a shift registercircuit in which signals having waveforms shifted by ¼ wavelength of theclock signals are output can be formed.

When the flip-flop circuit has a structure in which an EDMOS circuitthat is a combination of an enhancement type TFT and a depletion typeTFT is provided in the logic circuit portion and a dual-gate TFT isprovided in the switch portion, the amount of current flowing in theTFTs included in the logic circuit portion 361 can be increased and anarea occupied by the TFTs and furthermore, an area occupied by thecircuit including the TFTs can be reduced without reduction inperformance. Further, in the TFT included in the switch portion 362, theamount of current flowing in the TFTs can be increased and switching ofon/off can be performed at high speed; therefore, an area occupied bythe TFTs and furthermore, an area occupied by the circuit including theTFTs can be reduced without reduction in performance. Accordingly, anarrower frame, downsizing, high performance of a display device can beachieved.

Further, a latch circuit, a level shifter circuit, or the like can beprovided in the signal line driver circuit illustrated in FIGS. 3A and3B. A buffer portion is provided in the last stage through which asignal is transmitted from the signal line driver circuit to the pixelportion, and an amplified signal is transmitted from the signal linedriver circuit to the pixel portion. Thus, when a TFT having large oncurrent, typically a dual-gate TFT or a depletion type TFT is providedin the buffer portion, an area of the TFT can be reduced and an areaoccupied by the signal line driver circuit can be reduced. Accordingly,a narrow frame, downsizing, and high performance of a display device canbe achieved. Note that since high-speed operation is required for theshift register which is part of the signal line driver circuit, theshift register is preferably mounted on a display device by use of an ICor the like.

In addition, this embodiment can be freely combined with Embodiment 1 orEmbodiment 2.

Embodiment 4

In Embodiment 4, a method for manufacturing a display device includingthe second thin film transistor 170 described in Embodiment 1 will bedescribed with reference to FIGS. 9A to 9C, FIGS. 10A to 10C, FIG. 11,FIG. 12, FIG. 13, FIG. 14, FIGS. 15A, 15B, 15C, and 15D, and FIG. 16.

In FIG. 9A, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like can be used as a substrate 100having a light-transmitting property.

Next, after a conductive layer is formed over an entire surface of thesubstrate 100, a resist mask is formed by a first photolithography step.Then, unnecessary portions are removed by etching, thereby formingwirings and electrodes (a gate wiring including the gate electrode 101,a capacitor wiring 108, and a first terminal 121). At this time, etchingis performed so that at least an end portion of the gate electrode 101is tapered. A cross-sectional view at this stage is illustrated in FIG.9A. Note that FIG. 11 is a top view at this stage.

The gate wiring including the gate electrode 101, the capacitor wiring108, and the first terminal 121 in the terminal portion are desirablyformed of a low-resistant conductive material such as aluminum (Al) orcopper (Cu). However, aluminum itself has the disadvantages of low heatresistance, being easily corroded, and the like; thus, it is used incombination with a conductive material having heat resistance. As theconductive material having heat resistance, it is possible to use anelement selected from titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), analloy containing any of these elements as its component, an alloy filmcontaining a combination of any of these elements, or a nitridecontaining any of these elements as its component.

Then, the gate insulating layer 102 is entirely formed over the gateelectrode 101. The gate insulating layer 102 is formed to a thickness of50 nm to 400 nm by a sputtering method or the like. When yield of thethin film transistor is prioritized, the thickness of the gateinsulating layer 102 is preferably large.

For example, as the gate insulating layer 102, a silicon oxide film isformed to a thickness of 100 nm by a sputtering method. It is needlessto say that the gate insulating layer 102 is not limited to such asilicon oxide film, and another insulating film such as a siliconoxynitride film, a silicon nitride film, an aluminum oxide film, analuminum nitride film, an aluminum oxynitride film, or a tantalum oxidefilm may be used to form a single-layer structure or a stackedstructure. When a silicon oxynitride film, a silicon nitride film, orthe like is used as the gate insulating layer 102, an impurity from theglass substrate, sodium for example, can be preventing from diffusinginto and entering an oxide semiconductor to be formed later.

Note that dust adhering to the surface of the gate insulating layer ispreferably removed by reverse sputtering where plasma is generated byintroduction of an argon gas, before the oxide semiconductor film isformed. In addition, nitrogen, helium or the like may be used instead ofan argon atmosphere. Alternatively, the reserve sputtering may beconducted in an argon atmosphere to which oxygen, hydrogen, N₂O and/orthe like are/is added. Still alternatively, it may be conducted in anargon atmosphere to which Cl₂, CF₄, and/or the like are/is added.

Next, the first oxide semiconductor film (first In—Ga—Zn—O-basednon-single-crystal film in this embodiment) is formed over the gateinsulating layer 102. The first In—Ga—Zn—O-based non-single-crystal filmformed without being exposed to air after the plasma treatment, canavoid the trouble that dust or moisture adheres to the interface betweenthe gate insulating layer and the semiconductor film. Here, theIn—Ga—Zn—O-based non-single-crystal film is formed in an argonatmosphere or an oxygen atmosphere under the condition where the targetis an oxide semiconductor target including In (indium), Ga (gallium),and Zn (zinc) (In₂O₃:Ga₂O₃:ZnO=1:1:1) with a diameter of 8 inches, thedistance between the substrate and the target is set at 170 mm, thepressure is set at 0.4 Pa, and the direct current (DC) power supply isset at 0.5 kW. Note that a pulse direct current (DC) power supply ispreferable because dust can be reduced and the film thickness can beuniform. The thickness of the first In—Ga—Zn—O-based non-single-crystalfilm is set to from 5 nm to 200 nm. The thickness of the firstIn—Ga—Zn—O-based non-single-crystal film in this embodiment is 100 nm.

Next, a second oxide semiconductor film (second In—Ga—Zn—O-basednon-single-crystal film in this embodiment) is formed by a sputteringmethod without being exposed to air. Here, sputtering deposition isperformed under the condition where a target includes indium oxide(In₂O₃), gallium oxide (Ga₂O₃), and zinc oxide (ZnO) at a ratio of 1:1:1(═In₂O₃:Ga₂O₃:ZnO), the pressure in a deposition chamber is set at 0.4Pa, the electric power is set at 500 W, the deposition temperature isset to room temperature, and the argon gas flow rate is set at 40 sccm.Although the target of In₂O₃:Ga₂O₃:ZnO=1:1:1 is used intentionally, anIn—Ga—Zn—O-based non-single-crystal film including a crystal grain whichhas a size of 1 nm to 10 nm just after the film formation is obtained insome cases. It can be said that control of the presence or absence ofcrystal grains and the density of crystal grains and adjustment of thediameter of the crystal grain within 1 nm to 10 nm can be done byadjusting as appropriate, the deposition condition of reactivesputtering, such as the target composition ratio, the depositionpressure (0.1 Pa to 2.0 Pa), the electric power (250 W to 3000 W: 8inchesφ), the temperature (room temperature to 100° C.), or the like.The thickness of the second In—Ga—Zn—O-based non-single-crystal film isset to 5 nm to 20 nm Needless to say, the size of a grain included inthe film does not exceed the film thickness. In this embodiment, thesecond In—Ga—Zn—O-based non-single-crystal film has a thickness of 5 nm.

The first In—Ga—Zn—O-based non-single-crystal film is formed under thedifferent conditions from the second In—Ga—Zn—O-based non-single-crystalfilm. For example, as compared with the oxygen gas flow rate and theargon gas flow rate in the deposition condition of the secondIn—Ga—Zn—O-based non-single-crystal film, the oxygen gas flow rate inthe deposition condition of the first In—Ga—Zn—O-basednon-single-crystal film is increased. Specifically, the secondIn—Ga—Zn—O-based non-single-crystal film is formed in a rare gas (suchas argon or helium) atmosphere (or a gas including oxygen at 10% or lessand argon at 90% or more), while the first In—Ga—Zn—O-basednon-single-crystal film is formed in an oxygen atmosphere (or the oxygengas flow rate is equal to or larger than the argon gas flow rate).

The second In—Ga—Zn—O-based non-single-crystal film may be formed in thesame chamber as the chamber where the reverse sputtering is performedpreviously, or may be formed in a different chamber from the chamberwhere the reverse sputtering is performed previously.

Examples of sputtering include an RF sputtering in which ahigh-frequency power source is used for a sputtering power source, a DCsputtering, and a pulsed DC sputtering in which a bias is applied in apulsed manner.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can bedeposited to be stacked in the same chamber, or a plurality of kinds ofmaterials can be deposited by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering, and asputtering apparatus used for an ECR sputtering in which plasmagenerated with use of microwaves is used without using glow discharge.

Furthermore, as a deposition method by sputtering, there are also areactive sputtering in which a target substance and a sputtering gascomponent are chemically reacted with each other during deposition toform a thin compound film thereof, and a bias sputtering in whichvoltage is also applied to a substrate during deposition.

Next, a second photolithography step is performed to form a resist mask,and the first In—Ga—Zn—O-based non-single-crystal film and the secondIn—Ga—Zn—O-based non-single-crystal film are etched. Here, unnecessaryportions are removed by wet etching using ITO07N (manufactured by KANTOCHEMICAL CO., INC.), thereby forming the oxide semiconductor film 109that is the first In—Ga—Zn—O-based non-single-crystal film and an oxidesemiconductor film 111 that is the second In—Ga—Zn—O-basednon-single-crystal film. Note that this etching step may be dry etchingwithout being limited to wet etching. A cross-sectional view at thisstage is illustrated in FIG. 9B. Note that FIG. 12 is a top view at thisstage.

Next, a third photolithography step is conducted to form a resist mask,and unnecessary portions are removed by etching to form a contact holewhich reaches an electrode layer or a wiring made of the same materialsas the gate electrode layer. The contact hole is provided for directconnection with a conductive film to be formed later. For example, inthe driving circuit portion, a contact hole is formed when a thin filmtransistor whose gate electrode layer is direct contact with the sourceor drain electrode layer or a terminal that is electrically connected toa gate wiring of a terminal portion is formed. In this embodiment, theexample of forming the contact hole for direct connection with theconductive film to be formed later by the third photolithography step isdescribed, but there is no particular limitation and the contact holewhich reaches the gate electrode layer may be formed later at the sameprocess as a contact hole for connection with the pixel electrode, andelectrical connection may be performed with use of the same material asthe pixel electrode. In the case where electrical connection isperformed with use of the same material as the pixel electrode, one maskcan be reduced.

Then, a conductive film 132 formed from a metal material is formed overthe oxide semiconductor film 109 and the oxide semiconductor film 111 bya sputtering method or a vacuum evaporation method. A cross-sectionalview at this stage is illustrated in FIG. 9C.

As the material of the conductive film 132, there are an elementselected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any ofthese elements as its component, an alloy containing a combination ofany of these elements, and the like. If heat treatment at 200° C. to600° C. is performed, the conductive film preferably has heat resistanceenough to withstand the heat treatment. Since aluminum itself has thedisadvantages of low heat resistance, being easily corroded, and thelike, it is used in combination with a conductive material having heatresistance. As the conductive material having heat resistance, which iscombined with aluminum, it is possible to use an element selected fromtitanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium(Cr), neodymium (Nd), and scandium (Sc), an alloy containing any ofthese elements as its component, an alloy containing a combination ofany of these elements, or a nitride containing any of these elements asits component.

Here, the conductive film 132 has a single-layer structure of a titaniumfilm. The conductive film 132 may also have a two-layer structure inwhich a titanium film is stacked on an aluminum film. Alternatively, theconductive film 132 may have a three-layer structure in which a titanium(Ti) film, an aluminum film containing neodymium (Nd) (an Al—Nd film),and a titanium (Ti) film are stacked in order. Further alternatively,the conductive film 132 may have a single-layer structure of an aluminumfilm containing silicon.

Next, a fourth photolithography step is performed to form a resist mask131, and unnecessary portions are removed by etching to form the sourceand drain electrode layers 105 a and 105 b, the n⁺ layers 104 a and 104b serving as the source and drain regions, and a connection electrode120. This etching step is performed by wet etching or dry etching. Forexample, when an aluminum film or an aluminum alloy film is used as theconductive film 132, wet etching can be performed using a solution inwhich phosphoric acid, acetic acid, and nitric acid are mixed. Here,with use of an ammonia hydrogen peroxide mixture (hydrogenperoxide:ammonia:water=5:2:2), the conductive film 132 made of titanium(Ti) is wet-etched to form the source and drain electrode layers 105 aand 105 b, and the oxide semiconductor film 111 is wet-etched to formthe n⁺ layers 104 a and 104 b. In this etching step, an exposed regionof the oxide semiconductor film 109 is partially etched to be an oxidesemiconductor layer 103. In this manner, a channel region of the oxidesemiconductor layer 103 between the n⁺ layers 104 a and 104 b is aregion whose thickness is smaller. In FIG. 10A, formation of the sourceand drain electrode layers 105 a and 105 b and the n⁺ layers 104 a and104 b by etching are simultaneously conducted using an ammonia hydrogenperoxide mixture; therefore, the end portions of the source and drainelectrode layers 105 a and 105 b are aligned with the end portions ofthe n⁺ layers 104 a and 104 b, so that the end portions are continuous.In addition, wet etching allows the layers to be etched isotropically,so that the end portions of the source and drain electrode layers 105 aand 105 b are recessed from the resist mask 131. Through the abovesteps, the second thin film transistor 170 including the oxidesemiconductor layer 103 as its channel formation region can be formed. Across-sectional view at this stage is illustrated in FIG. 10A. Note thatFIG. 13 is a top view at this stage.

Then, heat treatment is preferably performed at 200° C. to 600° C., andtypically, 300° C. to 500° C. Here, heat treatment at 350° C. for onehour is performed in a furnace in a nitrogen atmosphere. This heattreatment involves the rearrangement at the atomic level in theIn—Ga—Zn—O-based non-single-crystal film. The heat treatment (includinglight annealing) in this step is important because the strain thatinhibits the movement of carriers can be released. Note that there is noparticular limitation on the timing of the heat treatment, and the heattreatment may be performed at any time after the deposition of thesecond In—Ga—Zn—O-based non-single-crystal film, for example, after theformation of a pixel electrode.

Furthermore, the exposed channel formation region of the oxidesemiconductor layer 103 may be subjected to oxygen radical treatment, sothat a normally-off thin film transistor can be obtained. In addition,the radical treatment can repair damage due to the etching of the oxidesemiconductor layer 103. The radical treatment is preferably performedin an atmosphere of O₂ or N₂O, and preferably an atmosphere of N₂, He,or Ar each containing oxygen. The radical treatment may also beperformed in an atmosphere in which Cl₂ and/or CF₄ is added to the aboveatmosphere. Note that the radical treatment is preferably performed withno bias applied.

In the fourth photolithography step, a second terminal 122 that is madeof the same material as the source and drain electrode layers 105 a and105 b is left in the terminal portion. Note that the second terminal 122is electrically connected to a source wiring (a source wiring includingthe source and drain electrode layers 105 a and 105 b).

In addition, in the terminal portion, the connection electrode 120 isdirectly connected to the first terminal 121 of the terminal portionthrough a contact hole formed in the gate insulating film. Note thatalthough not illustrated here, a source or drain wiring of the thin filmtransistor of the driver circuit is directly connected to the gateelectrode through the same steps as the above-described steps.

Further, by use of a resist mask having regions with plural thicknesses(typically, two different thicknesses) which is formed using amulti-tone mask, the number of resist masks can be reduced, resulting insimplified process and lower costs.

Next, the resist mask 131 is removed, and a protective insulating layer107 is formed to cover the second thin film transistor 170. For theprotective insulating layer 107, a single layer or a stacked layer of asilicon nitride film, a silicon oxide film, a silicon oxynitride film,an aluminum oxide film, an aluminum nitride film, an aluminum oxynitridefilm, a tantalum oxide film, and/or the like, which are/is obtained by asputtering method or the like can be used. In the thin film transistorin part of the driver circuit, the protective insulating layer 107functions as a second gate insulating layer and a second gate electrodeis formed thereover. The protective insulating layer 107 has a thicknessof 50 nm to 400 nm. When yield of the thin film transistor isprioritized, the thickness of the protective insulating layer 107 ispreferably large. Further, when a silicon oxynitride film, a siliconnitride film, or the like is used as the protective insulating layer107, impurities attached for some reason after the formation of theprotective insulating layer 107, e.g., sodium, can be prevented fromdiffusing into and entering the oxide semiconductor.

Then, a fifth photolithography step is performed to form a resist mask,and the protective insulating layer 107 is etched to form a contact hole125 reaching the drain electrode layer 105 b. In addition, a contacthole 126 reaching the connection electrode 120 and a contact hole 127reaching the second terminal 122 are also formed by this etching. Across-sectional view at this stage is illustrated in FIG. 10B.

Next, the resist mask is removed, and then a transparent conductive filmis formed. The transparent conductive film is formed of indium oxide(In₂O₃), indium oxide-tin oxide alloy (In₂O₃—SnO₂, abbreviated to ITO),or the like by a sputtering method, a vacuum evaporation method, or thelike. Such a material is etched with a hydrochloric acid-based solution.However, since a residue is easily generated particularly in etchingITO, indium oxide-zinc oxide alloy (In₂O₃—ZnO) may be used to improveetching processability.

Next, a sixth photolithography step is performed to form a resist mask,and unnecessary portions are removed by etching, thereby forming thepixel electrode 110 in the pixel portion. In the sixth photolithographystep, in the driver circuit, the same material as that of the pixelelectrode 110 is used for part of the circuit to form an electrode layer(a back-gate electrode) for controlling the threshold value over theoxide semiconductor layer. Note that the thin film transistor having theback-gate electrode is described in Embodiment 1 with reference to FIG.1A; therefore, detailed description thereof is omitted here.

In the sixth photolithography step, a storage capacitor is formed fromthe capacitor wiring 108 and the pixel electrode 110 by using the gateinsulating layer 102 and the protective insulating layer 107 in thecapacitor portion as dielectrics. Note that an example in which thestorage capacitor is formed from the capacitor wiring 108 and the pixelelectrode 110 by using the gate insulating layer 102 and the protectiveinsulating layer 107 as the dielectrics is described here. However,there is no particular limitation and a structure may also be employedin which an electrode formed of the same material as the sourceelectrode or the drain electrode is provided above the capacitor wiringand a storage capacitor is formed from the electrode and the capacitorwiring by using the gate insulating layer 102 therebetween as adielectric, thereby electrically connecting the electrode and the pixelelectrode.

Furthermore, in the sixth photolithography step, the first terminal andthe second terminal are covered with the resist mask so that transparentconductive films 128 and 129 are left in the terminal portion. Thetransparent conductive films 128 and 129 function as electrodes orwirings connected to an FPC. The transparent conductive film 128 formedover the connection electrode 120 which is directly connected to thefirst terminal 121 is a connection terminal electrode which functions asan input terminal of the gate wiring. The transparent conductive film129 formed over the second terminal 122 is a connection terminalelectrode which functions as an input terminal of the source wiring.

Then, the resist mask is removed. A cross-sectional view at this stageis illustrated in FIG. 10C. Note that FIG. 14 is a top view at thisstage.

FIGS. 15A and 15B respectively illustrate a cross-sectional view and atop view of a gate wiring terminal portion at this stage. FIG. 15A is across-sectional view taken along line C1-C2 of FIG. 15B. In FIG. 15A, atransparent conductive film 155 formed over a protective insulating film154 is a connection terminal electrode functioning as an input terminal.Further, in the terminal portion of FIG. 15A, a first terminal 151 madeof the same material as the gate wiring and a connection electrode 153made of the same material as the source wiring overlap with each otherwith a gate insulating layer 152 therebetween, and are in direct contactwith each other so as to be electrically connected. In addition, theconnection electrode 153 and the transparent conductive film 155 are indirect contact with each other through a contact hole provided in theprotective insulating film 154 so as to be electrically connected.

FIGS. 15C and 15D respectively illustrate a cross-sectional view and atop view of a source wiring terminal portion. FIG. 15C is across-sectional view taken along line D1-D2 of FIG. 15D. In FIG. 15C,the transparent conductive film 155 formed over the protectiveinsulating film 154 is a connection terminal electrode functioning as aninput terminal. Further, in the terminal portion of FIG. 15C, anelectrode 156 made of the same material as the gate wiring is formedbelow a second terminal 150 which is electrically connected to thesource wiring and overlaps with the second terminal 150 with the gateinsulating layer 152 interposed therebetween. The electrode 156 is notelectrically connected to the second terminal 150, and a capacitor toprevent noise or static electricity can be formed if the potential ofthe electrode 156 is set to a potential different from that of thesecond terminal 150, such as floating, GND, or 0 V. The second terminal150 is electrically connected to the transparent conductive film 155with the protective insulating film 154 therebetween.

A plurality of gate wirings, source wirings, and capacitor wirings areprovided depending on the pixel density. Also in the terminal portion,the first terminal at the same potential as the gate wiring, the secondterminal at the same potential as the source wiring, the third terminalat the same potential as the capacitor wiring, and the like are eacharranged in plurality. The number of each of the terminals may be anynumber, and the number of the terminals may be determined by apractitioner as appropriate.

Through these six photolithography steps, the second thin filmtransistor 170 which is a bottom-gate n-channel thin film transistor andthe storage capacitor can be completed using the six photomasks. Bydisposing the thin film transistor and the storage capacitor in eachpixel of a pixel portion in which pixels are arranged in a matrix form,one of substrates for manufacturing an active matrix display device canbe obtained. In this specification, such a substrate is referred to asan active matrix substrate for convenience.

When electrical connection to the gate wiring by using the same materialas the pixel electrode is conducted, the third photolithography step canbe omitted. Therefore, through the five photolithography steps, thesecond thin film transistor which is a bottom-gate n-channel thin filmtransistor and the storage capacitor can be completed using the fivephotomasks.

Further, when a material of the second gate electrode is different froma material of the pixel electrode as illustrated in FIG. 1C, onephotolithography step is added, so that one photomask is added.

In the case of manufacturing an active matrix liquid crystal displaydevice, an active matrix substrate and a counter substrate provided witha counter electrode are bonded to each other with a liquid crystal layerinterposed therebetween. Note that a common electrode electricallyconnected to the counter electrode on the counter substrate is providedover the active matrix substrate, and a fourth terminal electricallyconnected to the common electrode is provided in the terminal portion.The fourth terminal is provided so that the common electrode is set to afixed potential such as GND or 0 V.

Further, the pixel structure is not limited to that of FIG. 14, and anexample of a top view which is different from FIG. 14 is illustrated inFIG. 16. FIG. 16 illustrates an example in which a capacitor wiring isnot provided but a pixel electrode overlaps with a gate wiring of anadjacent pixel, with a protective insulating film and a gate insulatinglayer therebetween to form a storage capacitor. In that case, thecapacitor wiring and the third terminal connected to the capacitorwiring can be omitted. Note that in FIG. 16, the same parts as those inFIG. 14 are denoted by the same reference numerals.

In an active matrix liquid crystal display device, pixel electrodesarranged in a matrix form are driven to form a display pattern on ascreen. Specifically, voltage is applied between a selected pixelelectrode and a counter electrode corresponding to the pixel electrode,so that a liquid crystal layer provided between the pixel electrode andthe counter electrode is optically modulated and this optical modulationis recognized as a display pattern by an observer.

In displaying moving images, a liquid crystal display device has aproblem that a long response time of liquid crystal molecules themselvescauses afterimages or blurring of moving images. In order to improve themoving-image characteristics of a liquid crystal display device, adriving method called black insertion is employed in which black isdisplayed on the whole screen every other frame period.

Alternatively, a driving method called double-frame rate driving may beemployed in which the vertical cycle is 1.5 or 2 times as long as usualto improve the moving-image characteristics.

Further alternatively, in order to improve the moving-imagecharacteristics of a liquid crystal display device, a driving method maybe employed, in which a plurality of LEDs (light-emitting diodes) or aplurality of EL light sources are used to form a surface light source asa backlight, and each light source of the surface light source isindependently driven in a pulsed manner in one frame period. As thesurface light source, three or more kinds of LEDs may be used and an LEDemitting white light may be used. Since a plurality of LEDs can becontrolled independently, the light emission timing of LEDs can besynchronized with the timing at which a liquid crystal layer isoptically modulated. According to this driving method, LEDs can bepartly turned off; therefore, an effect of reducing power consumptioncan be obtained particularly in the case of displaying an image having alarge part on which black is displayed.

By combining these driving methods, the display characteristics of aliquid crystal display device, such as moving-image characteristics, canbe improved as compared to those of conventional liquid crystal displaydevices.

The n-channel transistor obtained in this embodiment uses anIn—Ga—Zn—O-based non-single-crystal film for its channel formationregion and has favorable dynamic characteristics. Accordingly, thesedriving methods can be applied in combination with the n-channeltransistor of this embodiment.

In manufacturing a light-emitting display device, one electrode (alsoreferred to as a cathode) of an organic light-emitting element is set toa low power supply potential such as GND or 0 V; thus, a terminalportion is provided with a fourth terminal for setting the cathode to alow power supply potential such as GND or 0 V. Also in manufacturing alight-emitting display device, a power supply line is provided inaddition to a source wiring and a gate wiring. Accordingly, the terminalportion is provided with a fifth terminal electrically connected to thepower supply line.

With use of the thin film transistor using the oxide semiconductor in agate line driver circuit or a source line driver circuit, manufacturingcost is reduced. Then, by directly connecting a gate electrode of thethin film transistor used in the driver circuit with a source wiring ora drain wiring, the number of contact holes can be reduced, so that adisplay device in which an area occupied by the driver circuit isreduced can be provided.

Accordingly, by applying this embodiment, a display device withexcellent electric characteristics can be provided at lower cost.

This embodiment can be freely combined with any of Embodiment 1,Embodiment 2, and Embodiment 3.

Embodiment 5

In Embodiment 5, an example of electronic paper as a semiconductordevice will be described.

FIG. 17 illustrates active matrix electronic paper as an example, whichis different from a liquid crystal display device. A thin filmtransistor 581 used in a pixel portion of the semiconductor device canbe formed in a manner similar to the thin film transistor of the pixelportion described in Embodiment 4 and is a thin film transistorincluding an In—Ga—Zn—O-based non-single-crystal film as a semiconductorlayer. In addition, as described in Embodiment 1, the pixel portion anda driver circuit can be formed over the same substrate, and therebyelectronic paper with low manufacturing cost can be realized.

The electronic paper in FIG. 17 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black or whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and apotential difference is generated between the first electrode layer andthe second electrode layer to control orientation of the sphericalparticles, so that display is performed.

The thin film transistor 581 is a thin film transistor with a bottomgate structure provided, and a source or drain electrode layer thereofis in contact with a first electrode layer 587 in an opening formed ininsulating layers 583, 584, 585, whereby the thin film transistor 581 iselectrically connected to the first electrode layer 587. Between thefirst electrode layer 587 and a second electrode layer 588, sphericalparticles 589 each having a black region 590 a, a white region 590 b,and a cavity 594 filled with liquid, around the regions, are providedbetween a pair of substrates 580 and 596. A space around the sphericalparticles 589 is filled with a filler 595 such as a resin (see FIG. 17).

Further, instead of the twisting ball, an electrophoretic element canalso be used. A microcapsule having a diameter of about 10 μm to 200 μmin which transparent liquid, positively charged white microparticles,and negatively charged black microparticles are encapsulated, is used.In the microcapsule which is provided between the first electrode layerand the second electrode layer, when an electric field is applied by thefirst electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides, sothat white or black can be displayed. A display element using thisprinciple is an electrophoretic display element and is called electronicpaper. The electrophoretic display element has higher reflectance than aliquid crystal display element, and thus, an auxiliary light isunnecessary, power consumption is low, and a display portion can berecognized in a dim place. In addition, even when power is not suppliedto the display portion, an image which has been displayed once can bemaintained. Accordingly, a displayed image can be stored even if asemiconductor device having a display function (which may be referred tosimply as a display device or a semiconductor device provided with adisplay device) is distanced from an electric wave source.

Through this process, electronic paper, which can be manufactured atlower cost, as a semiconductor device can be manufactured.

This embodiment can be combined with any content of Embodiment 1 orEmbodiment 2 as appropriate.

Embodiment 6

In Embodiment 6, an example of a light-emitting display device as asemiconductor device will be described. As a display element included ina display device, a light-emitting element utilizing electroluminescenceis described here. Light-emitting elements utilizing electroluminescenceare classified according to whether a light-emitting material is anorganic compound or an inorganic compound. In general, the former isreferred to as an organic EL element, and the latter is referred to asan inorganic EL element.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element as a light-emitting element is described here.

FIG. 18 illustrates an example of a pixel structure to which digitaltime grayscale driving can be applied, as an example of a semiconductordevice.

A structure and operation of a pixel to which digital time grayscaledriving can be applied are described. In this embodiment, one pixelincludes two n-channel transistors each of which includes an oxidesemiconductor layer (an In—Ga—Zn—O-based non-single-crystal film) as itschannel formation region.

A pixel 6400 includes a switching transistor 6401, a driver transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driver transistor 6402. The gate of the driver transistor 6402 isconnected to a power supply line 6407 via the capacitor 6403, a firstelectrode of the driver transistor 6402 is connected to the power supplyline 6407, and a second electrode of the driver transistor 6402 isconnected to a first electrode (pixel electrode) of the light-emittingelement 6404. A second electrode of the light-emitting element 6404corresponds to a common electrode 6408.

The second electrode (common electrode 6408) of the light-emittingelement 6404 is set to a low power supply potential. Note that the lowpower supply potential is a potential satisfying the low power supplypotential <a high power supply potential with reference to the highpower supply potential that is set to the power supply line 6407. As thelow power supply potential, GND, 0 V, or the like may be employed, forexample. A potential difference between the high power supply potentialand the low power supply potential is applied to the light-emittingelement 6404 and current is supplied to the light-emitting element 6404,so that the light-emitting element 6404 emits light. Here, in order tomake the light-emitting element 6404 emit light, each potential is setso that the potential difference between the high power supply potentialand the low power supply potential is a forward threshold voltage orhigher of the light-emitting element 6404.

Note that gate capacitor of the driver transistor 6402 may be used as asubstitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitor of the driver transistor 6402 may be formedbetween the channel region and the gate electrode.

In the case of a voltage-input voltage driving method, a video signal isinput to the gate of the driver transistor 6402 so that the drivertransistor 6402 is in either of two states of being sufficiently turnedon and turned off. That is, the driver transistor 6402 operates in alinear region. Since the driver transistor 6402 operates in the linearregion, a voltage higher than the voltage of the power supply line 6407is applied to the gate of the driver transistor 6402. Note that avoltage higher than or equal to (voltage of the power supply line+Vth ofthe driver transistor 6402) is applied to the signal line 6405.

In a case of performing analog grayscale driving instead of digital timegrayscale driving, the same pixel structure as that in FIG. 18 can beused by changing signal input.

In the case of performing analog grayscale driving, a voltage higherthan or equal to (forward voltage of the light-emitting element 6404+Vthof the driver transistor 6402) is applied to the gate of the drivertransistor 6402. The forward voltage of the light-emitting element 6404indicates a voltage at which a desired luminance is obtained, andincludes at least forward threshold voltage. The video signal by whichthe driver transistor 6402 operates in a saturation region is input, sothat current can be supplied to the light-emitting element 6404. Inorder for the driver transistor 6402 to operate in the saturationregion, the potential of the power supply line 6407 is set higher thanthe gate potential of the driver transistor 6402. When an analog videosignal is used, it is possible to feed current to the light-emittingelement 6404 in accordance with the video signal and perform analoggrayscale driving.

Note that the pixel structure illustrated in FIG. 18 is not limitedthereto. For example, a switch, a resistor, a capacitor, a transistor, alogic circuit, or the like may be added to the pixel illustrated in FIG.18.

Next, structures of the light-emitting element will be described withreference to FIGS. 19A to 19C. A cross-sectional structure of a pixelwill be described by taking a case where the driving TFT is the thinfilm transistor 170 illustrated in FIG. 1B as an example. Driving TFTs7001, 7011, and 7021 used for semiconductor devices illustrated in FIGS.19A to 19C can be formed in a manner similar to the thin film transistor170 described in Embodiment 1 and are thin film transistors whichinclude In—Ga—Zn—O-based non-single-crystal films as their semiconductorlayers and which have excellent electric characteristics.

In order to extract light emitted from the light-emitting element, atleast one of an anode and a cathode is required to transmit light. Athin film transistor and a light-emitting element are formed over asubstrate. A light-emitting element can have a top emission structure,in which light emission is extracted through the surface opposite to thesubstrate; a bottom emission structure, in which light emission isextracted through the surface on the substrate side; or a dual emissionstructure, in which light emission is extracted through the surfaceopposite to the substrate and the surface on the substrate side. Thepixel structure illustrated in FIG. 18 can be applied to alight-emitting element having any of these emission structures.

A light-emitting element having a top emission structure will bedescribed with reference to FIG. 19A.

FIG. 19A is a cross-sectional view of a pixel in the case where thedriving TFT 7001 is the thin film transistor 170 illustrated in FIG. 1Band light is emitted from a light-emitting element 7002 to an anode 7005side. In FIG. 19A, a cathode 7003 of the light-emitting element 7002 iselectrically connected to the driving TFT 7001, and a light-emittinglayer 7004 and the anode 7005 are stacked in this order over the cathode7003. The cathode 7003 can be formed using a variety of conductivematerials as long as they have a low work function and reflect light.For example, Ca, Al, MgAg, AlLi, or the like is preferably used. Thelight-emitting layer 7004 may be formed using a single layer or aplurality of layers stacked. When the light-emitting layer 7004 isformed using a plurality of layers, the light-emitting layer 7004 isformed by stacking an electron-injecting layer, an electron-transportinglayer, a light-emitting layer, a hole-transporting layer, and ahole-injecting layer in this order over the cathode 7003. It is notnecessary to form all of these layers. The anode 7005 is formed using alight-transmitting conductive film such as a film of indium oxideincluding tungsten oxide, indium zinc oxide including tungsten oxide,indium oxide including titanium oxide, indium tin oxide includingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

The light-emitting element 7002 corresponds to a region where thelight-emitting layer 7004 is sandwiched between the cathode 7003 and theanode 7005. In the case of the pixel illustrated in FIG. 19A, light isemitted from the light-emitting element 7002 to the anode 7005 side asindicated by an arrow.

A second gate electrode provided over the oxide semiconductor layer inthe driver circuit is preferably formed from the same material as thecathode 7003, which leads to simplification of the process.

Next, a light-emitting element having a bottom emission structure willbe described with reference to FIG. 19B. FIG. 19B is a cross-sectionalview of a pixel in the case where the driving TFT 7011 is the thin filmtransistor 170 illustrated in FIG. 1A and light is emitted from alight-emitting element 7012 to a cathode 7013 side. In FIG. 19B, thecathode 7013 of the light-emitting element 7012 is formed over alight-transmitting conductive film 7017 that is electrically connectedto the driving TFT 7011, and a light-emitting layer 7014 and an anode7015 are stacked in this order over the cathode 7013. A light-blockingfilm 7016 for reflecting or blocking light may be formed to cover theanode 7015 when the anode 7015 has a light-transmitting property. Forthe cathode 7013, a variety of materials can be used as in the case ofFIG. 19A as long as they are conductive materials having a low workfunction. The cathode 7013 is formed to have a thickness that cantransmit light (preferably, approximately 5 nm to 30 nm). For example,an aluminum film with a thickness of 20 nm can be used as the cathode7013. Similar to the case of FIG. 19A, the light-emitting layer 7014 maybe formed using either a single layer or a plurality of layers stacked.The anode 7015 is not required to transmit light, but can be formedusing a light-transmitting conductive material as in the case of FIG.19A. As the light-blocking film 7016, a metal or the like that reflectslight can be used for example; however, it is not limited to a metalfilm. For example, a resin or the like to which black pigments are addedcan also be used.

The light-emitting element 7012 corresponds to a region where thelight-emitting layer 7014 is sandwiched between the cathode 7013 and theanode 7015. In the case of the pixel illustrated in FIG. 19B, light isemitted from the light-emitting element 7012 to the cathode 7013 side asindicated by an arrow.

A second gate electrode provided over the oxide semiconductor layer inthe driver circuit is preferably formed from the same material as thecathode 7013, which leads to simplification of the process.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 19C. In FIG. 19C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. As in the case of FIG. 19A, thecathode 7023 can be formed using a variety of conductive materials aslong as they have a low work function. The cathode 7023 is formed tohave a thickness that can transmit light. For example, a film of Alhaving a thickness of 20 nm can be used as the cathode 7023. As in FIG.19A, the light-emitting layer 7024 may be formed using either a singlelayer or a plurality of layers stacked. The anode 7025 can be formedusing a light-transmitting conductive material as in the case of FIG.19A.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 overlapwith one another. In the case of the pixel illustrated in FIG. 19C,light is emitted from the light-emitting element 7022 to both the anode7025 side and the cathode 7023 side as indicated by arrows.

A second gate electrode provided over the oxide semiconductor layer inthe driver circuit is preferably formed from the same material as theconductive film 7027, which leads to simplification of the process.Further, the second gate electrode provided over the oxide semiconductorlayer in the driver circuit is preferably formed with a stack of thesame materials as the conductive film 7027 and the cathode 7023, therebylowering wiring resistance as well as simplification of the process.

Note that, although the organic EL elements are described here as thelight-emitting elements, an inorganic EL element can also be provided asa light-emitting element.

In this embodiment, the example is described in which a thin filmtransistor (a driving TFT) which controls the driving of alight-emitting element is electrically connected to the light-emittingelement; however, a structure may be employed in which a TFT for currentcontrol is connected between the driving TFT and the light-emittingelement.

A semiconductor device described in this embodiment is not limited tothe structures illustrated in FIGS. 19A to 19C and can be modified invarious ways based on the spirit of techniques disclosed.

Next, a top view and a cross section of a light-emitting display panel(also referred to as a light-emitting panel), which is one embodiment ofa semiconductor device, will be described with reference to FIGS. 20Aand 20B. FIG. 20A is the top view of a panel in which a thin filmtransistor and a light-emitting element formed over a first substrateare sealed between the first substrate and a second substrate with asealant. FIG. 20B is a cross-sectional view taken along a line H-I ofFIG. 20A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. It is preferable that a panel be packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air, in this manner.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors.In FIG. 20B, a thin film transistor 4510 included in the pixel portion4502 and a thin film transistor 4509 included in the signal line drivercircuit 4503 a are illustrated as an example.

For each of the thin film transistors 4509 and 4510, a highly reliablethin film transistor including an In—Ga—Zn—O-based non-single-crystalfilm as its semiconductor layer as described in Embodiment 1 can beapplied. In addition, the thin film transistor 4509 includes gateelectrodes above and below the semiconductor layer as described inEmbodiment 1 with reference to FIG. 1B.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that a structure of the light-emitting element 4511 is astacked-layer structure of the first electrode layer 4517, theelectroluminescent layer 4512, and the second electrode layer 4513, butthere is no particular limitation on the structure. The structure of thelight-emitting element 4511 can be changed as appropriate depending onthe direction in which light is extracted from the light-emittingelement 4511, or the like.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, or organic polysiloxane. It is particularly preferablethat the partition 4520 be formed using a photosensitive material and anopening be formed over the first electrode layer 4517 so that a sidewallof the opening is formed as an inclined surface with continuouscurvature.

The electroluminescent layer 4512 may be formed with a single layer or aplurality of layers stacked.

A protective film may be formed over the second electrode layer 4513 andthe partition 4520 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4511. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

In addition, a variety of signals and potentials are supplied to thesignal line driver circuits 4503 a and 4503 b, the scan line drivercircuits 4504 a and 4504 b, or the pixel portion 4502 from FPCs 4518 aand 4518 b.

In this embodiment, a connection terminal electrode 4515 is formed fromthe same conductive film as the first electrode layer 4517 included inthe light-emitting element 4511, and a terminal electrode 4516 is formedfrom the same conductive film as the source and drain electrode layersincluded in the thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a via an anisotropic conductive film4519.

The second substrate 4506 located in the direction in which light isextracted from the light-emitting element 4511 should have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used for the second substrate 4506.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, PVC (polyvinyl chloride), acrylic, polyimide, anepoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylenevinyl acetate) can be used.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be provided by mounting drivercircuits formed using a single crystal semiconductor film orpolycrystalline semiconductor film over a single-crystal substrate or aninsulating substrate separately prepared. In addition, only the signalline driver circuits or part thereof, or the scan line driver circuitsor part thereof may be separately formed and mounted. This embodiment isnot limited to the structure illustrated in FIGS. 20A and 20B.

Through this process, a display device (display panel) can bemanufactured at lower manufacturing cost.

This embodiment can be combined with any content of Embodiment 1 orEmbodiment 2 as appropriate.

Embodiment 7

In this embodiment, a top view and a cross section of a liquid crystaldisplay panel, which is one embodiment of a semiconductor device, willbe described with reference to FIGS. 21A, 21B and 21C. FIGS. 21A and 21Bare top views of a panel in which highly reliable thin film transistors4010 and 4011 each including the In—Ga—Zn—O-based non-single-crystalfilm as its semiconductor layer as described in Embodiment 1 and aliquid crystal element 4013, which are formed over a first substrate4001 are sealed between the first substrate 4001 and a second substrate4006 with a sealant 4005. FIG. 21C is a cross-sectional view taken alonga line M-N of FIGS. 21A and 21B.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Therefore, the pixelportion 4002 and the scan line driver circuit 4004 are sealed togetherwith a liquid crystal layer 4008, by the first substrate 4001, thesealant 4005, and the second substrate 4006. A signal line drivercircuit 4003 that is formed using a single crystal semiconductor film ora polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that the connection method of a driver circuit which is separatelyformed is not particularly limited, and a COG method, a wire bondingmethod, a TAB method, or the like can be used. FIG. 21A illustrates anexample of mounting the signal line driver circuit 4003 by a COG method,and FIG. 21B illustrates an example of mounting the signal line drivercircuit 4003 by a TAB method.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 include a plurality of thin filmtransistors. FIG. 21C illustrates the thin film transistor 4010 includedin the pixel portion 4002 and the thin film transistor 4011 included inthe scan line driver circuit 4004. Over the thin film transistors 4010and 4011, insulating layers 4020 and 4021 are provided.

Each of the thin film transistors 4010 and 4011 can be the thin filmtransistor including an In—Ga—Zn—O-based non-single-crystal film as itssemiconductor layer described in Embodiment 1. The thin film transistor4011 corresponds to the thin film transistor having a back gateelectrode described in Embodiment 2 with reference to FIG. 2A.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is provided forthe second substrate 4006. A portion where the pixel electrode layer4030, the counter electrode layer 4031, and the liquid crystal layer4008 overlap with one another corresponds to the liquid crystal element4013. Note that the pixel electrode layer 4030 and the counter electrodelayer 4031 are provided with an insulating layer 4032 and an insulatinglayer 4033 respectively which each function as an alignment film, andthe liquid crystal layer 4008 is sandwiched between the pixel electrodelayer 4030 and the counter electrode layer 4031 with the insulatinglayers 4032 and 4033 therebetween.

Note that the first substrate 4001 and the second substrate 4006 can beformed of glass, metal (typically, stainless steel), ceramic, orplastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. In addition, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films can beused.

Reference numeral 4035 denotes a columnar spacer obtained by selectivelyetching an insulating film and is provided to control the distancebetween the pixel electrode layer 4030 and the counter electrode layer4031 (a cell gap). Further, a spherical spacer may also be used. Inaddition, the counter electrode layer 4031 is electrically connected toa common potential line formed over the same substrate as the thin filmtransistor 4010. With use of the common connection portion, the counterelectrode layer 4031 and the common potential line can be electricallyconnected to each other by conductive particles arranged between a pairof substrates. Note that the conductive particles are included in thesealant 4005.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase is generated within an onlynarrow range of temperature, liquid crystal composition containing achiral agent at 5 wt % or more so as to improve the temperature range isused for the liquid crystal layer 4008. The liquid crystal compositionwhich includes liquid crystal exhibiting a blue phase and a chiral agenthave such characteristics that the response time is 10 μs to 100 μs,which is short, the alignment process is unnecessary because the liquidcrystal composition has optical isotropy, and viewing angle dependencyis small.

Although the example of a transmissive liquid crystal display device isdescribed in this embodiment, one embodiment of the present inventioncan also be applied to a reflective liquid crystal display device and atransflective liquid crystal display device.

While an example of the liquid crystal display device in which thepolarizing plate is provided on the outer side of the substrate (on theviewer side) and the coloring layer and the electrode layer used for adisplay element are provided on the inner side of the substrate in thatorder is described in this embodiment, the polarizing plate may beprovided on the inner side of the substrate. The stacked structure ofthe polarizing plate and the coloring layer is not limited to thisembodiment and may be set as appropriate depending on materials of thepolarizing plate and the coloring layer or conditions of manufacturingprocess. Further, a light-blocking film serving as a black matrix may beprovided.

In this embodiment, in order to reduce surface unevenness of the thinfilm transistor and to improve reliability of the thin film transistor,the thin film transistor obtained in Embodiment 1 is covered with theinsulating layers (the insulating layer 4020 and the insulating layer4021) functioning as a protective film or a planarizing insulating film.Note that the protective film is provided to prevent entry ofcontaminant impurities such as organic substance, metal, or moistureexisting in air and is preferably a dense film. The protective film maybe formed with a single layer or a stacked layer of a silicon oxidefilm, a silicon nitride film, a silicon oxynitride film, a siliconnitride oxide film, an aluminum oxide film, an aluminum nitride film,aluminum oxynitride film, and/or an aluminum nitride oxide film by asputtering method. Although an example in which the protective film isformed by a sputtering method is described in this embodiment, there isno particular limitation and the protective film may be formed by avariety of methods such as PCVD. In part of the driver circuit, theprotective film serves as a second gate insulating layer, and a thinfilm transistor having a back gate over the second gate insulating layeris provided.

In this embodiment, the insulating layer 4020 having a stacked-layerstructure is formed as a protective film. Here, as a first layer of theinsulating layer 4020, a silicon oxide film is formed by a sputteringmethod. The use of a silicon oxide film as a protective film has aneffect of preventing hillock of an aluminum film.

As a second layer of the protective film, an insulating layer is formed.In this embodiment, as the second layer of the insulating layer 4020, asilicon nitride film is formed by a sputtering method. The use of thesilicon nitride film as the protective film can prevent mobile ions ofsodium or the like from entering a semiconductor region and changingelectrical characteristics of the TFT.

After the protective film is formed, the semiconductor layer may besubjected to annealing (300° C. to 400° C.). In addition, the back gateis formed after the protective film is formed.

The insulating layer 4021 is formed as the planarizing insulating film.As the insulating layer 4021, an organic material having heat resistancesuch as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can beused. Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like. Note that the insulating layer 4021 may be formed by stackinga plurality of insulating films formed of these materials.

Note that a siloxane-based resin is a resin formed from a siloxanematerial as a starting material and having the bond of Si—O—Si. Thesiloxane-based resin may include as a substituent an organic group(e.g., an alkyl group or an aryl group) or a fluoro group. In addition,the organic group may include a fluoro group.

A formation method of the insulating layer 4021 is not particularlylimited, and the following method can be employed depending on thematerial: a sputtering method, an SOG method, a spin coating method, adipping method, a spray coating method, a droplet discharge method(e.g., an ink-jet method, screen printing, offset printing, or thelike), a doctor knife, a roll coater, a curtain coater, a knife coater,or the like. In a case of forming the insulating layer 4021 using amaterial solution, annealing (300° C. to 400° C.) of the semiconductorlayer may be performed at the same time as a baking step. The bakingstep of the insulating layer 4021 also serves as annealing of thesemiconductor layer, whereby a semiconductor device can be manufacturedefficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide including tungsten oxide, indium zinc oxide including tungstenoxide, indium oxide including titanium oxide, indium tin oxide includingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, indium tin oxide to which silicon oxide is added, orthe like.

Conductive compositions including a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed using the conductive composition preferably has a sheetresistance of less than or equal to 10000 ohms per square and atransmittance of greater than or equal to 70% at a wavelength of 550 nm.Further, the resistivity of the conductive high molecule included in theconductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called t-electron conjugatedconductive polymer can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of two or more kinds of them, and thelike can be given.

Further, a variety of signals and potentials are supplied to the signalline driver circuit 4003 which is formed separately, the scan linedriver circuit 4004, or the pixel portion 4002 from an FPC 4018.

In this embodiment, a connection terminal electrode 4015 is formed fromthe same conductive film as that of the pixel electrode layer 4030included in the liquid crystal element 4013, and a terminal electrode4016 is formed from the same conductive film as that of the source anddrain electrode layers of the thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 via an anisotropic conductive film4019.

FIGS. 21A and 21B illustrate an example in which the signal line drivercircuit 4003 is formed separately and mounted on the first substrate4001; however, this embodiment is not limited to this structure. Thescan line driver circuit may be separately formed and then mounted, oronly part of the signal line driver circuit or part of the scan linedriver circuit may be separately formed and then mounted.

FIG. 22 illustrates an example in which a liquid crystal display moduleis formed as a semiconductor device by using a TFT substrate 2600.

FIG. 22 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTor the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the RGB system, respective coloring layers corresponding tocolors of red, green, and blue are provided for respective pixels.Polarizing plates 2606 and 2607 and a diffusion plate 2613 are providedoutside the TFT substrate 2600 and the counter substrate 2601. A lightsource includes a cold cathode tube 2610 and a reflective plate 2611,and a circuit substrate 2612 is connected to a wiring circuit portion2608 of the TFT substrate 2600 by a flexible wiring board 2609 andincludes an external circuit such as a control circuit or a power sourcecircuit. The polarizing plate and the liquid crystal layer may bestacked with a retardation plate therebetween.

For the liquid crystal display module, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, amulti-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, an axially symmetric aligned micro-cell (ASM)mode, an optical compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC)mode, or the like can be used.

Through this process, a liquid crystal display device as a semiconductordevice can be manufactured at lower manufacturing cost.

This embodiment can be combined with any content of Embodiment 1, 2 or 3as appropriate.

Embodiment 8

A semiconductor device according to one embodiment of the presentinvention can be applied to a variety of electronic devices (includingan amusement machine). Examples of electronic devices are a televisionset (also referred to as a television or a television receiver), amonitor of a computer or the like, a camera such as a digital camera ora digital video camera, a digital photo frame, a mobile phone handset(also referred to as a mobile phone or a mobile phone device), aportable game console, a portable information terminal, an audioreproducing device, a large-sized game machine such as a pachinkomachine, and the like.

FIG. 23A illustrates an example of a portable information terminaldevice 9200. The portable information terminal device 9200 incorporatesa computer and thus can process various types of data. An example of theportable information terminal device 9200 is a personal digitalassistant.

The portable information terminal device 9200 has two housings, ahousing 9201 and a housing 9203. The housing 9201 and the housing 9203are joined with a joining portion 9207 such that the portableinformation terminal device 9200 can be foldable. A display portion 9202is incorporated in the housing 9201, and the housing 9203 includes akeyboard 9205. Needless to say, the structure of the portableinformation terminal device 9200 is not limited to the above structure,and the structure may include at least a thin film transistor having aback gate electrode, and additional accessory may be provided asappropriate. A driver circuit and a pixel portion are formed over thesame substrate, which leads to reduction of the manufacturing cost.Thus, a portable information terminal device having a thin filmtransistor having high electric characteristics can be realized.

FIG. 23B illustrates an example of a digital video camera 9500. Thedigital video camera 9500 includes a display portion 9503 incorporatedin a housing 9501 and various operation portions. Needless to say, thestructure of the digital video camera 9500 is not limited to the abovestructure, and the structure may include at least a thin film transistorhaving a back gate electrode, and additional accessory may be providedas appropriate. A driver circuit and a pixel portion are formed over thesame substrate, which leads to reduction of the manufacturing cost.Thus, a digital video camera having a thin film transistor having highelectric characteristics can be realized.

FIG. 23C illustrates an example of a mobile phone 9100. The mobile phone9100 has two housings, a housing 9102 and a housing 9101. The housing9102 and the housing 9101 are joined with a joining portion such thatthe mobile phone is foldable. A display portion 9104 is incorporated inthe housing 9102, and the housing 9101 includes operation keys 9106.Needless to say, the structure of the mobile phone 9100 is not limitedto the above structure, and the structure may include at least a thinfilm transistor having a back gate electrode, and additional accessorymay be provided as appropriate. A driver circuit and a pixel portion areformed over the same substrate, which leads to reduction of themanufacturing cost. Thus, a mobile phone having a thin film transistorhaving high electric characteristics can be realized.

FIG. 23D illustrates an example of a portable computer 9400. Thecomputer 9400 has two housings, a housing 9401 and a housing 9404. Thehousing 9401 and the housing 9404 are joined such that the computer canbe open and closed. A display portion 9402 is incorporated in thehousing 9401, and the housing 9404 includes a key board 9403 or thelike. Needless to say, the structure of the computer 9400 is notparticularly limited to the above structure, and the structure mayinclude at least a thin film transistor having a back gate electrode,and additional accessory may be provided as appropriate. A drivercircuit and a pixel portion are formed over the same substrate, whichleads to reduction of the manufacturing cost. Thus, a computer having athin film transistor having high electric characteristics can berealized.

FIG. 24A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. The display portion 9603 can display images. Further, the housing9601 is supported by a stand 9605 in FIG. 24A.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with an operation key 9609 of the remote controller9610 so that an image displayed on the display portion 9603 can becontrolled. Further, the remote controller 9610 may be provided with adisplay portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the receiver, a general television broadcast can bereceived. Further, when the television set 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver or between receivers) data communication canbe performed.

FIG. 24B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. The display portion 9703 can display avariety of images. For example, the display portion 9703 can displaydata of an image taken with a digital camera or the like and function asa normal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection portion (a USB terminal, a terminal thatcan be connected to various cables such as a USB cable, or the like), arecording medium insertion portion, and the like. Although thesecomponents may be provided on the surface on which the display portionis provided, it is preferable to provide them on the side surface or theback surface for the design of the digital photo frame 9700. Forexample, a memory storing data of an image taken with a digital camerais inserted in the recording medium insertion portion of the digitalphoto frame, whereby the image data can be transferred and thendisplayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. The structure may be employed in which desired imagedata is transferred wirelessly to be displayed.

FIG. 25A illustrates an example of a mobile phone 1000 which isdifferent from that illustrated in FIG. 23C. The mobile phone 1000includes a display portion 1002 incorporated in a housing 1001, anoperation button 1003, an external connection port 1004, a speaker 1005,a microphone 1006 and the like.

In the mobile phone 1000 illustrated in FIG. 25A, data can be input whena person touches the display portion 1002 with his/her finger or thelike. In addition, operations such as phone call or mailing can beconducted when a person touches the display portion 1002 with his/herfinger or the like.

There are mainly three screen modes of the display portion 1002: thefirst mode is a display mode mainly for displaying an image; the secondmode is an input mode mainly for inputting data such as text; and thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are combined.

For example, in a case of calling or mailing, a text input mode mainlyfor inputting text is selected for the display portion 1002 so that textdisplayed on a screen can be input. In that case, it is preferable todisplay a keyboard or number buttons on almost all area of the screen ofthe display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 1000, display in the screen of the display portion 1002 canbe automatically switched by determining the installation direction ofthe mobile phone 1000 (whether the mobile phone 1000 is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 1002 oroperating the operation button 1003 of the housing 1001. Alternatively,the screen modes may be switched depending on the kind of the imagedisplayed on the display portion 1002. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 1002 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken when thedisplay portion 1002 is touched with a palm or a finger, wherebypersonal identification can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

FIG. 25B illustrates another example of a mobile phone. The mobile phonein FIG. 25B has a display device 9410 in a housing 9411, which includesa display portion 9412 and operation buttons 9413, and a communicationdevice 9400 in a housing 9401, which includes operation buttons 9402, anexternal input terminal 9403, a microphone 9404, a speaker 9405, and alight-emitting portion 9406 that emits light when a phone call isreceived. The display device 9410 which has a display function can bedetached from or attached to the communication device 9400 which has aphone function, in two directions represented by the allows. Thus, thedisplay device 9410 and the communication device 9400 can be attached toeach other along their short sides or long sides. In addition, when onlythe display function is needed, the display device 9410 can be detachedfrom the communication device 9400 and used alone. Images or inputinformation can be transmitted or received by wireless or wirecommunication between the communication device 9400 and the displaydevice 9410, each of which has a rechargeable battery.

Embodiment 9

In this embodiment, an example of a display device including a thin filmtransistor in which a wiring and an oxide semiconductor layer are incontact with each other is described with reference to FIG. 26. It is tobe noted that the same portions in FIG. 26 as those in FIG. 1A will bedescribed with use of the same reference numerals.

A first thin film transistor 480 illustrated in FIG. 26 is a thin filmtransistor used in a driver circuit, in which a first wiring 409 and asecond wiring 410 are provided in contact with an oxide semiconductorlayer 405. The first thin film transistor 480 includes a first gateelectrode 401 below the oxide semiconductor layer 405 and a second gateelectrode 470 above the oxide semiconductor layer 405.

In addition, a second thin film transistor 481 is a thin film transistorused in a pixel portion, in which source or drain electrode layers 105 aand 105 b are provided in contact with the oxide semiconductor layer103.

The semiconductor device of this embodiment has a structure in which thewiring and the oxide semiconductor layer are in contact with each other,and thus the number of steps can be reduced as compared with that ofEmbodiment 1.

This embodiment can be combined with any structure of the otherembodiments.

This application is based on Japanese Patent Application serial no.2008-274540 filed with Japanese Patent Office on Oct. 24, 2008, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising: forming a first conductive film that serves as afirst gate electrode of a transistor over a substrate; forming a firstinsulating film over the first conductive film; forming an oxidesemiconductor layer that comprises a channel formation region over thefirst insulating film; forming a second insulating film over the oxidesemiconductor layer; and forming a transparent film that comprises anoxide including indium and zinc over the second insulating film by asputtering method, wherein the transparent film serves as a second gateelectrode of the transistor, wherein the first conductive film and thetransparent film overlap each other with the oxide semiconductor layertherebetween, and wherein the transparent film has a larger width thanthe oxide semiconductor layer in a channel width direction of thetransistor.
 2. The method according to claim 1, wherein the oxidesemiconductor layer includes indium, zinc, and a metal different fromindium and zinc.
 3. The method according to claim 1, wherein thetransparent film is a conductive film.
 4. The method according to claim1, further comprising: forming a source electrode and a drain electrodeby patterning a second conductive film formed over the oxidesemiconductor layer, whereby a top surface of a first region of theoxide semiconductor layer that is located between the source electrodeand the drain electrode is partially etched to have a smaller thicknessthan a second region of the oxide semiconductor layer that is below thesource electrode or the drain electrode.
 5. The method according toclaim 1, wherein the transparent film has a larger width than the oxidesemiconductor layer in a channel length direction of the transistor. 6.The method according to claim 1, further comprising: performing a heattreatment on the oxide semiconductor layer.
 7. The method according toclaim 1, further comprising: forming a second oxide semiconductor layerover the oxide semiconductor layer without exposing the oxidesemiconductor layer to air.
 8. A method for manufacturing asemiconductor device, comprising: forming a first conductive film thatserves as a first gate electrode of a transistor over a substrate;forming a first insulating film over the first conductive film; formingan oxide semiconductor layer that comprises a channel formation regionover the first insulating film; forming a source electrode and a drainelectrode that are electrically connected to the oxide semiconductorlayer; forming a second insulating film over the oxide semiconductorlayer, the source electrode, and the drain electrode; and forming atransparent film that comprises an oxide including indium and zinc overthe second insulating film by a sputtering method, wherein thetransparent film serves as a second gate electrode of the transistor,wherein the first conductive film and the transparent film overlap eachother with the oxide semiconductor layer therebetween, and wherein thetransparent film has a larger width than the oxide semiconductor layerboth in a channel length direction and a channel width direction of thetransistor.
 9. The method according to claim 8, wherein the oxidesemiconductor layer includes indium, zinc, and a metal different fromindium and zinc.
 10. The method according to claim 8, wherein thetransparent film is a conductive film.
 11. The method according to claim8, wherein the source electrode and the drain electrode are formed bypatterning a second conductive film formed over the oxide semiconductorlayer, whereby a top surface of a first region of the oxidesemiconductor layer that is located between the source electrode and thedrain electrode is partially etched to have a smaller thickness than asecond region of the oxide semiconductor layer that is below the sourceelectrode or the drain electrode.
 12. The method according to claim 8,further comprising: performing a heat treatment on the oxidesemiconductor layer.
 13. The method according to claim 8, furthercomprising: forming a second oxide semiconductor layer over the oxidesemiconductor layer without exposing the oxide semiconductor layer toair.
 14. The method according to claim 8, wherein the second insulatingfilm is an oxide insulating film.
 15. The method according to claim 14,wherein the second insulating film is in contact with the oxidesemiconductor layer and the transparent film.
 16. The method accordingto claim 14, wherein the second insulating film is a silicon oxide film.17. A method for manufacturing a semiconductor device, comprising:forming a first conductive film that serves as a first gate electrode ofa transistor over a substrate; forming a first insulating film over thefirst conductive film; forming an oxide semiconductor layer thatcomprises a channel formation region over the first insulating film;forming a source electrode and a drain electrode by patterning a secondconductive film formed over the oxide semiconductor layer, whereby a topsurface of a first region of the oxide semiconductor layer that islocated between the source electrode and the drain electrode is etchedto have a smaller thickness than a second region of the oxidesemiconductor layer that is below the source electrode or the drainelectrode; forming a second insulating film over the oxide semiconductorlayer, the source electrode, and the drain electrode; and forming atransparent film that comprises an oxide including indium and zinc overthe second insulating film by a sputtering method, wherein thetransparent film serves as a second gate electrode of the transistor,wherein the first conductive film and the transparent film overlap eachother with the oxide semiconductor layer therebetween.
 18. The methodaccording to claim 17, wherein the oxide semiconductor layer includesindium, zinc, and a metal different from indium and zinc.
 19. The methodaccording to claim 17, wherein the transparent film is a conductivefilm.
 20. The method according to claim 17, further comprising:performing a heat treatment on the oxide semiconductor layer.
 21. Themethod according to claim 17, further comprising: forming a second oxidesemiconductor layer over the oxide semiconductor layer without exposingthe oxide semiconductor layer to air.
 22. The method according to claim17, wherein the second insulating film is an oxide insulating film. 23.The method according to claim 22, wherein the second insulating film isin contact with the oxide semiconductor layer and the transparent film.24. The method according to claim 22, wherein the second insulating filmis a silicon oxide film.
 25. A method for manufacturing a semiconductordevice, comprising: forming a first conductive film that serves as afirst gate electrode of a transistor over a substrate; forming a firstinsulating film over the first conductive film; forming an oxidesemiconductor layer that comprises a channel formation region over thefirst insulating film; forming a second insulating film over the oxidesemiconductor layer; and forming a transparent film that comprises anoxide including indium and zinc over the second insulating film by asputtering method, wherein the transparent film serves as a second gateelectrode of the transistor, wherein the first conductive film and thetransparent film overlap each other with the oxide semiconductor layertherebetween, wherein the transparent film has a larger width than theoxide semiconductor layer in a channel width direction of thetransistor, and wherein the second insulating film is an oxideinsulating film.
 26. The method according to claim 25, wherein the oxidesemiconductor layer includes indium, zinc, and a metal different fromindium and zinc.
 27. The method according to claim 25, wherein thetransparent film is a conductive film.
 28. The method according to claim25, further comprising: forming a source electrode and a drain electrodeby patterning a second conductive film formed over the oxidesemiconductor layer, whereby a top surface of a first region of theoxide semiconductor layer that is located between the source electrodeand the drain electrode is partially etched to have a smaller thicknessthan a second region of the oxide semiconductor layer that is below thesource electrode or the drain electrode.
 29. The method according toclaim 25, wherein the transparent film has a larger width than the oxidesemiconductor layer in a channel length direction of the transistor. 30.The method according to claim 25, further comprising: performing a heattreatment on the oxide semiconductor layer.
 31. The method according toclaim 25, further comprising: forming a second oxide semiconductor layerover the oxide semiconductor layer without exposing the oxidesemiconductor layer to air.
 32. The method according to claim 25,wherein the second insulating film is in contact with the oxidesemiconductor layer and the transparent film.
 33. The method accordingto claim 25, wherein the second insulating film is a silicon oxide film.